WASHINGTON - Countering military assertions that refusals to take the anthrax vaccine are having little impact, a congressional study finds it is the leading cause mentioned by pilots and aircrew members for leaving National Guard and reserve units.

The Pentagon questioned the results yesterday, saying guard and reserve strength and readiness is unaffected. But officers acknowledged they have no data on how many reservists are leaving rather than taking the shots.

Combined with congressional testimony from former and present reservists claiming persecution for refusing the vaccine, the General Accounting Office survey adds to congressional pressure on the Pentagon to give up its beleaguered vaccination program. A vaccine shortage has limited the program to forces in East Asia and the Persian Gulf area.

Commercial pilot Tom Heemstra, who complained in congressional testimony a year ago about having to take the anthrax vaccine as a squadron commander in the Indiana Air National Guard, estimated that 2,100 pilots from the Air National Guard and Air Force Reserves would be lost if the vaccination program continues.

Heemstra, of Lexington, Ky., who said he was forced to retire for his refusal, said anthrax has caused more than 200 resignations at several bases around the country. He provided the House Government Reform Committee with a list of coded names of some pilots who have left, saying military authorities are falsely reporting the numbers of departures linked to the vaccine.

Accusing Pentagon officers of abusing their power, he said, "They coerced, intimidated, threatened and punished in order to enforce this program."

Committee Chairman Dan Burton, an Indiana Republican holding his second anthrax hearing this month, accused the director of the Air National Guard of lying about the effect of the vaccine on departures from the guard, and suggested that Maj. Gen. Paul A. Weaver Jr. should face court-martial.

"The Defense Department has insulted the honor and integrity of anyone who has dared question the anthrax vaccine program," Burton said.

Weaver responded at the hearing by saying he never meant to mislead Congress or guard members during a closed-circuit briefing on anthrax at which he defended a previous declaration that only one guardsman had refused the vaccine. Weaver said this did not include those who had made no formal commitment to the guard and simply left, because guard service is entirely voluntary.

"It is very difficult to get an accurate picture," he said.

Maj. Gen. Randall West, the Pentagon's senior adviser on anthrax, told the committee that four of six reserve units show increased readiness in recent years. There are no indications that concern over the anthrax vaccine was a factor in the decreased readiness of the other two, he said. "While there have been individual cases of reserve component members refusing the vaccination, documented losses from such cases are a very small minority," he said.

West said the GAO survey focused exclusively on anthrax, while the military says that surveys of why people leave are more reliable if they do not mention anthrax and simply ask for reasons.

In the GAO survey, 25 percent of those who left their units, either through requested transfer or resignation, mentioned the mandatory anthrax immunization as the No. 1 factor in their decision. No other factor ranked higher, it said.

Also, 18 percent of those left in the units said they planned to leave during the next six months, with the vaccine also the leading factor mentioned. The anonymous survey said that 86 percent of respondents taking the vaccine reported "experiencing some type of local or systemic reactions," such as a knot in the arm or joint pain.

The data came in a preliminary version of a more exhaustive GAO report released for the hearing.