A statistical method similar to back projection was used to estimate the number of children infected with HIV perinatally from the number of children with AIDS due to perinatal HIV infection. By 31 December 1994 there were 18 children born in Australia and reported to have AIDS due to perinatal HIV infection. Based on these 18 AIDS cases, it was estimated that to the end of 1994 there were between 35 and 57 children born in Australia and infected with HIV perinatally, with a preferred estimate of 43 children. Assuming that the transmission rate of HIV from infected mothers to their babies was in the range 15 to 35%, it was estimated that the prevalence of HIV infection among women giving birth in Australia had increased from 0.03 per 1000 (range 0.02-0.07 per 1000) in 1983-1985 to 0.09 per 1000 (range 0.05-0.20 per 1000) in 1992-1994. This range for the prevalence of HIV infection among pregnant women in 1992-1994 was consistent with estimates of the prevalence from screening studies, and suggests that the prevalence of HIV infection in pregnant women in Australia remains low.