Moreover, the group averages themselves are based on assumptions that could well be inaccurate, the authors acknowledge, thus possibly throwing the risk estimates off by a factor of ten. But they stressed that this would not affect the overall conclusions about relative risks of different behaviors.

The San Francisco doctors calculated their estimates from existing data on the prevalence of the virus in various high- and low-risk groups, the risk of transmitting the virus in a single sexual encounter with an infected person and an assumption, admittedly a cautious one, that condoms were roughly 90 percent effective in preventing infection.

Their report found a ''tremendous variation'' in the degree of risk, depending on the circumstances, ranging from 1 in 5 billion for a low-risk partner to 1 in 500 in the most dangerous single encounters.

In a middle category, one-time sex with someone who is not in a high risk group but whose infection status is unknown carried a 1 in 5 million risk of infection without a condom. That is about the same as the risk of being killed in a traffic accident while driving 10 miles on the way to that sexual encounter. The odds of winning the current Lotto game sponsored by the New York State Lottery are 1 in 13 million. Most Dangerous Situations

At the most dangerous extremes among those considered, the report found that a single act of vaginal sex, without using a condom, with a partner who is known to be infected with the AIDS virus, carries a 1-in-500 risk of infection. And 500 acts of intercourse with such a partner raises the risk of infection to two in three.

Those estimates were based on transmission of the virus from men to women. There is some evidence that it is more difficult for women to transmit the virus to men, but the authors say, ''The size of this difference, if any, is unknown.''

Other experts said the risk estimates in the report appeared to be reasonable based on current knowledge but noted that they were based on very limited data, so they may be inaccurate.