The Rochester, Minn.-based clinic specified his condition as Bipolar II, which is defined as periodic episodes of depression and hypomania. Hypomania is a less serious form of mania.
"Congressman Jackson is responding well to the treatment and regaining his strength," the clinic said in a statement.
Bipolar II is a treatable condition that affects parts of the brain controlling emotion, thought and drive and is likely caused by a "complex set of genetic and environmental factors," the clinic said. The statement also mentioned that Jackson underwent weight loss treatment in 2004. The clinic says weight loss surgery can change how the body absorbs foods and medications.
Jackson spokesman Frank Watkins declined to comment on the statement. A Jackson aide said last week that the congressman was expected back in the district within a matter of weeks.
The announcement Monday was the most detailed to date about the congressman's mysterious medical leave which began June 10. His office initially described his medical condition as exhaustion and weeks later a statement from the office called it a "mood disorder."
It was only earlier this month that Jackson's office said he was at the Rochester, Minn., clinic and being treated for depression, after a transfer from the Sierra Tucson Treatment Center in Arizona.
Jackson, who first won office in 1995, is on the November ballot with two little-known candidates. He is widely expected to win re-election in the district that stretches south of Chicago and includes several suburbs.