OBJECTIVES: We evaluated changes in characteristics of clients presenting for voluntary counseling and testing (VCT) before and during care and treatment center (CTC) scale-up activities in Moshi, Tanzania, between November 2003 and December 2007. METHODS: Consecutive clients were surveyed after pretest counseling, and rapid HIV antibody testing was performed. Trend tests were used to assess changes in seroprevalence and client characteristics over time. Multivariable logistic regression models were used to estimate the contribution of changes in sociodemographic and behavioral risk characteristics, and symptoms, to changes in seroprevalence before and during CTC scale-up. RESULTS: Data from 4391 first-time VCT clients were analyzed. HIV seroprevalence decreased from 26.2% to 18.9% after the availability of free antiretroviral therapy and expansion of CTCs beyond regional and referral hospitals. Seroprevalence decreased by 27 % for females (P = 0.0002) and 34% for males (P = 0.0125). Declines in seropositivity coincided with decreases in symptoms among males and females (P < 0.0001) and a more favorable distribution of sociodemographic risks among females (P = 0.002). No changes in behavioral risk characteristics were observed. CONCLUSIONS: Concurrent with the scale-up of CTCs, HIV seroprevalence and rates of symptoms declined sharply at an established freestanding VCT site in Moshi, Tanzania. If more HIV-infected persons access VCT at sites where antiretrovirals are offered, freestanding VCT sites may become a less cost-effective means for HIV case finding