A top Defence Department health official told journalists on Wednesday that all but three of the 21 deaths were from the army.
Assistant secretary for defence for health affairs, William Winkenwered, said the army took the brunt of suicide fatalities because it was by far and away the largest service in Iraq.
The army's suicide rate, at 13.5 deaths per 100,000 soldiers per year, compared with a prior trend of about 11 in 100,000 before the war.
But Winkenwered played down the significance with statistics. Although it "is on the high side of what they have seen in the past, the real question is ... is that rate out of line?"
"Each one of these suicide events, unfortunate as they are, are investigated. But we don't see a trend there in looking at these cases that tells us there is more we might be doing, something different than we might be doing," he said.
No unusual trends were seen in the suicide rates for the other military services.
"The real question is ... is that [suicide] rate out of line?"
assistant secretary for defence for health affairs
Winkenwerder estimated that between 300 and 400 service members have been evacuated from Iraq for treatment for mental problems.
He had no statistics on the number of service members who have been treated for stress-related problems in Iraq.
Literature and past data also show that most people who suffer mental stress or mental illness do better with counselling, medication and treatment in the field, according to health officials.
The army has nine combat stress teams in Iraq, and the air force has two. In addition, a psychiatrist, a social worker and a psychologist is assigned to each division.
"We've put a lot into mental health resources in the field. We think that helps and has made a difference," Winkenwered said.