No details about where Jackson is being treated were included in the statement, which was released amid mounting pressure that he reveal his whereabouts and exact medical condition. Jackson went on leave June 10, though his office didn't disclose it until weeks later and has been mum on details ever since.
Staff members said the statement Wednesday was from Jackson's physician but that the doctor's name and location would not be released because of federal privacy laws.
"The Congressman is receiving intensive medical treatment at a residential treatment facility for a mood disorder," the statement said. "He is responding positively to treatment and is expected to make a full recovery."
Jackson's office said reports about the 47-year-old congressman being treated for "alcohol or substance abuse" weren't true. His spokesman declined to elaborate on the statement.
When Jackson's medical leave was first announced, his office said he was being treated for exhaustion. Last week his staff said his condition was worse than previously thought and required inpatient treatment, saying Jackson had been privately battling emotional problems.
The timing of the leave has raised questions, in part because Jackson is facing an ethics investigation in the U.S. House connected to imprisoned former Illinois Gov. Rod Blagojevich.
Earlier Wednesday, Democratic leaders in the U.S. House joined Jackson's colleagues and constituents in urging the congressman to provide a public update about his condition. House Leader Nancy Pelosi, when asked about Jackson, said she hoped he would have "the appropriate evaluation so he can share that information."
"I feel sad that whatever the situation is that he finds himself having to be away from Congress," Pelosi said. "Hopefully we'll see him back here soon again."
Several experts said that based on the doctor's use of the term "mood disorder," they believed Jackson might be suffering from depression. Doctors interviewed by The Associated Press who didn't have first-hand knowledge of Jackson's condition said the term typically refers to depression or bipolar disorder, which used to be known as manic depression.
Dr. Daniel Yohanna, vice chairman of the Department of Psychiatry and Behavioral Neuroscience at the University of Chicago, said depression is the more common and affects about 5 percent of men at some point in their lives. Symptoms can range from sleep disturbance and appetite problems to hopelessness and thoughts of suicide, though cure rates are very high, he said.
"It could come out of nowhere, it runs in families, you could have a genetic predisposition, or it can come after a difficulty in your life," he said. "Once it gets rolling it's hard to stop it on your own."
Ian Gotlib, a professor of psychology at Stanford University, said depression is generally treated on an outpatient basis. But if doctors were concerned about the safety of the patient or if the disorder were severe enough, they could recommend inpatient treatment.
"The good news is that it's clearly treatable," Gotlib said, adding that counseling and prescription drugs would be likely for inpatient treatment and that it could take weeks.
It's unclear whether Wednesday's statement would temper calls for more information.
Fellow Illinois Democrats Sen. Dick Durbin and Rep. Luis Gutierrez have called it Jackson's responsibility as a public official to disclose details. Jackson's little-known opponents in the November election have spoken out on the same issue, and voters in his district have asked questions.
Rep. Steny Hoyer of Maryland, the No. 2 Democrat in the U.S. House, said Wednesday that Jackson wasn't in "an unusual circumstance."
"People get sick, and when people get sick, they miss work. Everybody in America understands that," Hoyer said. "But I think the family would be well advised to give his constituents as much information as is appropriate."
Jackson spokesman Rick Bryant has said relatives requested Jackson's location be kept private, and his family has been unusually reticent on the issue. His wife has said little and Jackson's civil rights leader father, the Rev. Jesse Jackson Sr., has called it a private issue and repeatedly declined to give details.
A House Ethics Committee investigation is pending over allegations that Jackson discussed raising money for Blagojevich's campaign so the then-Illinois governor would appoint him to President Barack Obama's vacated U.S. Senate seat. Blagojevich is serving a prison sentence for corruption. Jackson has denied those claims.
Jackson also allegedly directed a fundraiser, Raghuveer Nayak, to buy plane tickets for a woman described as Jackson's "social acquaintance." Jackson and his wife have called that a personal matter.
Days before Jackson announced the medical leave, Nayak was arrested and pleaded not guilty to unrelated medical fraud charges. At Blagojevich's 2010 corruption trial, prosecutors said another Blagojevich fundraiser was ready to testify that Jackson instructed Nayak to raise money for Blagojevich's campaign to help him secure the Senate seat. The same witness later testified he attended a meeting with Jackson and Nayak.
Jackson was not charged and has repeatedly denied wrongdoing.
Jackson faces a Republican and independent candidate in November, though he's widely expected to win re-election. He first won office in a 1995 special election and has easily won each race since. Jackson's district includes parts of Chicago and some suburbs.
Associated Press writers Donna Cassata in Washington and Jason Keyser in Chicago contributed to this report.