We conducted a community-based study to determine the predictors of HIV-1 among women aged 20-44 years (N = 1,418) and their regular male partners (N = 566) from randomly selected households in Moshi, Tanzania. The weighted prevalence of HIV-1 was 10.3% in women and 7% in men. The highest risk of HIV-1 was in subjects whose partners were HIV-1 seropositive in both women (adjusted odds ratio (AOR) = 26.63; 95% confidence interval (CI): 10.74-66.02) and men (AOR = 22.25; 95%CI: 7.06-70.15). Herpes simplex virus type 2 (HSV-2) and Mycoplasma genitalium were also significantly associated with HIV-1. Women with male partners >or=12 years older than themselves had increased risk of HIV-1 (AOR = 1.99; 95%CI: 1.01-7.85). Other predictors of HIV-1 were history of infertility and the number of sex partners in the last three years in women and the age at time of circumcision and history of past sexually transmitted diseases (STDs) in male partners. These findings show that HIV-1/STDs were major public health problems among women and their long-term partners in this population. HIV-1 prevention efforts should include promotion of couple's HIV-1 counseling and testing services, control of HSV-2, promotion of safer sexual practices and strategies to reduce the age difference between women and their partners.