Their assessment was likely to become political ammunition for Arizona Sen. John McCain and other Republicans fighting to keep Congress from repealing the 1993 law that prohibits gays from acknowledging their sexual orientation. Democrats have promised a vote this month to repeal the "don't ask, don't tell" law, although its chances of passing this year were considered slim.
"If the law is changed, successfully implementing repeal and assimilating openly homosexual Marines into the tightly woven fabric of our combat units has strong potential for disruption at the small unit level, as it will no doubt divert leadership attention away from an almost singular focus of preparing units for combat," the Marine commandant, Gen. James Amos, told the Senate Armed Services Committee.
Army Chief of Staff Gen. George Casey, who led the Iraq war under President George W. Bush, was somewhat more optimistic. He said the policy shift, if implemented properly, wouldn't keep the Army from doing its job, and he predicted repeal would pose only a moderate risk to his force.
But, he added, changing the law now would "add another level of stress to any already stretched force" and be more difficult on the Army, particularly its combat units, than the recent Pentagon study suggests.
McCain said the testimony by the service chiefs should be given special consideration.
"It is the job of the service chiefs to ensure that our military is ready and able to win the nation's wars," he said at the beginning of Friday's hearing.
President Barack Obama has called on Congress to overturn the ban on openly gay service. Defense Secretary Robert Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, agreed and ordered a 10-month study looking at the attitudes of service members toward gay troops.
Released earlier this week, the study found that a minority of troops - about 30 percent - predicted potential problems if "don't ask, don't tell" were repealed.
But most of the troops with concerns were serving in combat roles. Nearly 60 percent of troops in the Marine Corps and in Army combat units, such as infantry and special operations, said they thought allowing gays to be open about their sexual orientation would hurt their units' ability to fight on the battlefield.
"I cannot reconcile, nor turn my back, on the negative perceptions held by our Marines who are most engaged in the hard work of day-to-day operations in Afghanistan," Amos said.
Gates and Mullen have said they believe resistance can be addressed through training and education. They also cite experience with gay troops as a mitigating factor. According to the study, 84 percent of Marines in combat roles who find they're working with a gay comrade said they did not see any negative impact on unit morale or cohesion.
"In terms of actual disruption experienced, as opposed to predicted disruption, the distinction between combat arms communities and the force as a whole is negligible," said Marine Gen. James Cartwright, the No. 2 officer on the Joint Chiefs of Staff.
The Air Force and Navy chiefs offered milder assessments of repeal than their Army and Marine counterparts. According to the study, some 70 percent of airmen and sailors predict few problems with lifting the ban.
The chief of naval operations, Adm. Gary Roughead, said it was likely that some highly trained combat sailors, including Navy SEALs, might refuse to re-enlist in protest of the personnel change. But, he said, he did not think any long-term damage would occur if certain steps were taken, such as increased training, and he recommended repeal.
The Air Force chief of staff, Gen. Norton Schwartz, predicted the risk to his force would be moderate. He suggested, however, deferring any policy changes until 2012 so as not to place any "additional discretionary demands on our leadership cadres in Afghanistan at this particularly challenging time."
McCain has dismissed the military study as flawed because it did not ask troops whether they thought the law should be repealed in the first place, focusing instead on the impact repeal might have. McCain also contends that Pentagon leadership was glossing over serious objections expressed by troops in Marine and Army combat roles at a time of two wars.
Cartwright counters in his testimony that implementing change at a time of war might actually be preferable because troops are focused on their mission.
"The challenges associated with making a change of any kind that seem enormous during periods of inactivity become less distracting when you are defending your nation and comrades," he said.
Associated Press writer Anne Gearan contributed to this report.