AIDS surveillance in Japan has recently witnessed an abrupt increase in the number of foreign females infected with HIV, which soared from 2 in 1989 to 274 in 1992. To clarify underlying causes for such a phenomenon anti-HIV seropositive rates and anti-HIV testing rates of foreign females who visited hospitals or clinics were investigated either by a retrospective medical records survey or by direct serological tests in several districts in the Tokyo metropolitan area. These studies revealed that seropositive rates of foreign females, mostly from Thailand, ranged between 1.0-5.0% during Jan. 1990-Apr. 1993, varying between districts and with a slight increase over the period. The proportion of foreign females who underwent anti-HIV test was found to have markedly increased from around 10% in 1990 to 75% in 1991 in spite of testing rates for syphilis or gonorrheal infection remaining unchanged during the same period. Their sexual behaviour was also studied and found that most of them were engaged in sex activities without using condom. These results suggest (1) The number of foreign females with HIV infection could have been considerably underestimated by the surveillance because in 1991, when the surveillance detected only 105 foreign females, around 600 or more foreign female infectees may have existed in Japan as estimated by multiplying the 3% infection rate with 20,000, the estimated female Thai population illegally residing in Japan by the end of 1991.(ABSTRACT TRUNCATED AT 250 WORDS)