While HIV counseling and testing (HCT) has been considered an HIV preventive measure in Africa, data are limited describing behavior changes following HCT. This study evaluated behavior changes and estimated HIV seroincidence rate among returning HCT clients. Repeat and one-time testing clients receiving HCT services in Moshi, Tanzania were identified. Information about sociodemographic characteristics, HIV-related behaviors and testing reasons were collected, along with HIV serostatus. Six thousand seven hundred and twenty-seven clients presented at least once for HCT; 1235 (18.4%) were HIV seropositive, median age was 29.7 years and 3712 (55.3%) were women. 1382 repeat and 4272 one-time testers were identified. Repeat testers were more likely to be male, older, married, or widowed, and testing because of unfaithful partner or new sexual partner. One-time testers were more likely to be students and testing due to illness. At second test, repeat testers were more likely to report that partners had received HIV testing, not have concurrent partners, not suspect partners have HIV, and have partners who did not have other partners. Clients who intended to change behaviors after the first test were more likely to report having changed behaviors by remaining abstinent (OR 2.58; p<0.0001) or using condoms (OR 2.00; p=0.006) at the second test. HIV seroincidence rate was 1.49 cases/100 person-years (PY). Clients presenting for repeat HCT reported some reduction of risky behavior and improved knowledge of sexual practices and HIV serostatus of their partners. Promoting behavior change through HCT should continue to be a focus of HIV prevention efforts in sub-Saharan Africa.