Few population-based studies have described the physical activity patterns of individuals trying to lose weight. We analysed cross-sectional, self-reported data from 6125 men and 12,557 women (> or = 18 years of age) from 40 states who reported in the 1989 Behavioral Risk Factor Surveillance System (BRFSS) survey that they were trying to lose weight. The BRFSS is a state-based telephone survey of non-institutionalized US adults, which utilizes a multi-stage cluster design based on the Waksberg method of random digit dialing. Walking was the most prevalent physical activity (48%), followed by aerobics (8%), gardening (5%), cycling (5%), and running (4%); however, activity choices varied by socio-demographic characteristics. Among both sexes, the prevalence of overweight (BMI > or = 30 kg/m2) decreased significantly with increasing level of activity (P < or = 0.001). Regression results showed that among both sexes and in most age groups, those who ran or jogged, performed aerobics, or cycled weighed less (P < or = 0.001) than those who reported no activity--independent of height, race, education, smoking, and caloric restriction. Walking was also associated with lower weight (P < or = 0.001) among persons aged 40 or older. Because walking is highly prevalent among persons in most socio-demographic strata and is accessible and low risk, its relative merits should be stressed in developing interventions for weight loss and maintenance.