BACKGROUND: Prevention of mother to child transmission of HIV (PMTCT) has been scaled, to more than 90% of health facilities in Tanzania. Disclosure of HIV results to partners and their participation is encouraged in the program. This study aimed to determine the prevalence, patterns and predictors of HIV sero-status disclosure to partners among HIV positive pregnant women in Morogoro municipality, Tanzania. METHODS: A cross sectional study was conducted in March to May 2010 among HIV-positive pregnant women who were attending for routine antenatal care in primary health care facilities of the municipality and had been tested for HIV at least one month prior to the study. Questionnaires were used to collect information on possible predictors of HIV disclosure to partners. RESULTS: A total of 250 HIV-positive pregnant women were enrolled. Forty one percent (102) had disclosed their HIV sero-status to their partners. HIV-disclosure to partners was more likely among pregnant women who were < 25 years old [Adjusted odds ratio (AOR) = 2.2; 95% CI: 1.2-4.1], who knew their HIV status before the current pregnancy [AOR = 3.7; 95% CI: 1.7-8.3], and discussed with their partner before testing [AOR = 6.9; 95% CI: 2.4-20.1]. Dependency on the partner for food/rent/school fees, led to lower odds of disclosure to partners [AOR = 0.4; 95% CI: 0.1-0.7]. Nine out of ten women reported to have been counseled on importance of disclosure and partner participation. CONCLUSIONS: Six in ten HIV positive pregnant women in this setting had not disclosed their results of the HIV test to their partners. Empowering pregnant women to have an individualized HIV-disclosure plan, strengthening of the HIV provider initiated counseling and testing and addressing economic development, may be some of the strategies in improving HIV disclosure and partner involvement in this setting.