BACKGROUND: Factors related to specific sexual encounters can influence condom use during these encounters. These situation-specific factors have not been adequately studied in resource-poor countries where HIV infection has in some areas reached epidemic levels. This study was undertaken to identify situation-specific factors associated with condom use among 465 female bar and hotel workers in Moshi, Tanzania. METHODS: We conducted a case-crossover study in which women provided information about their most recent unprotected and protected sexual encounters. Conditional logistic regression was used to estimate paired odds ratios and 95% confidence intervals for the association between situation-specific factors and condom use. RESULTS: A subject-based or mutual decision about condom use (compared with partner based), casual partner type, a first-time sexual encounter and receiving gifts in exchange for sex were independently associated with increased odds of condom use, while sex at home and sex with a partner more than 10 years older was associated with reduced odds of use. There was also effect modification between partner type and decision-making: subject-based or mutual decisions were more protective with casual than regular partners; also, when the partner made the decisions about condom use, the type of partner had no effect. CONCLUSIONS: Decision-making about condom use is a potentially modifiable predictor of unprotected sex, but its effect varies by partner type. Behavioural interventions are needed that encourage discussion about condom use and increase women's self-efficacy, but other types of interventions as well as female-controlled HIV prevention methods are needed for women in regular partnerships