The heterosexual transmission of subtype E human immunodeficiency virus type 1 (HIV-1) infection was evaluated in 467 couples in Thailand in whom the man was HIV-1 positive and the woman had no risk factors for HIV other than sex with her infected partner. At baseline, 216 (46.3%) of the 467 women were positive for HIV-1; prevalence of HIV among women was 52.2% when their male partners had CD4+ lymphocyte counts of <200 cells/microL, 45.9% in women whose partners had counts of 200-499 micro/L, and 39.2% in women whose partners had counts of >/=500/microL. Women were twice as likely to be HIV positive if their partners had a history of a sexually transmitted disease (STD); however, their HIV prevalence was 29% among couples who had no STD history. It appears that female partners of men infected with subtype E HIV-1 are at high risk of infection even when the man's CD4+ cell count is relatively high. A high rate of STDs may contribute significantly to this risk [corrected].