Masters, Johnson and Kolodny a volume which claims that risk understated by public This paper presents a brief analysis of the key data contained in MJK's book using simple mathematical models. The paper shows that the unconditional per partner risk of infection for the sexually active heterosexuals in the MJK study is exceeded by the similar risk for typical gay men in San Francisco by a factor of nearly fifty; that the conditional probability of infection given exposure via vaginal sex is about 2% for men and 3% for women per partner (both of these numbers are lower than the corresponding infec‐tivities for unprotected anal sex among gay men); and that an AIDS epidemic among the general heterosexual population would occur only if the average number of sexual partners per year exceeds about 5.3, an unlikely number given the data in MJK's study.