The mode of HIV-1 transmission was determined for 410 Saudi patients. Heterosexual transmission occurred in 189 patients (46%), blood product transfusion in 107 (26%), and perinatal transmission in 47 (12%). Men who have sex with men and injection drug users represented 5% and 2%,respectively. Sixty-three of 65 heterosexually infected women acquired HIV-1 from their spouses, whereas 111 of 124 heterosexually infected men acquired the virus from commercial sex workers. Heterosexual transmission is the main mode in Saudi patients.