Women's ability to negotiate safer sexual practices, particularly condom use, is a vital component of HIV/STD prevention strategies. Gender-based power imbalances may constrain women's negotiation ability, yet few empirical studies have tested the hypothesis that sexual relationship power constitutes a key factor in condom use negotiation. In this investigation, a new measure - the Sexual Relationship Power Scale (SRPS) - was applied. Data were collected from 388, mostly Latina, women at an urban community health centre in Massachusetts. Women with high levels of relationship power were five times as likely as women with low levels to report consistent condom use, after controlling for sociodemographic and psychosocial variables (p < 0.05). Population attributable risk estimates indicate that 52% of the lack of consistent condom use among women can be attributed to low sexual relationship power. The strong association between the Sexual Relationship Power Scale and consistent condom use supports the hypothesis that relationship power plays a key role in safer sex decision making. These findings underscore the importance of including the issue of relationship power in the design and implementation of programmes that promote sexual and reproductive health, as well as research investigating condom use and HIV risk.