OBJECTIVE: To determine associations of sociodemographic characteristics, behaviors, attitudes and medical factors with weight loss in a population-based biracial cohort of young adults. DESIGN: Two-year longitudinal observation study. SUBJECTS: 4278 black and white men and women aged 18-30 years in 1985-1986. MEASUREMENTS: Weight, height, education, income, subscapular skinfold thickness, waist and hip circumferences, medical history, smoking, physical activity, dietary energy, fat and alcohol intake, physical fitness measured by treadmill testing, self-reported dieting, history of weight loss and regain, perception of body size and belief in health consequences of overweight. RESULTS: Weight loss, defined as reduction of 5% or more of baseline weight, occurred in 8.9% of the cohort, ranging from 4.9% among white men who were not overweight at baseline to 21.5% among white women who were overweight. Weight loss was more common in women than men and in overweight white women than overweight black women. Although findings differed among the four race-sex groups, weight loss was generally associated with greater baseline fatness, lower baseline physical fitness level, self-perception of being overweight, dieting and previous weight loss and regain. Greater baseline fatness, a perception of being overweight and dieting were also associated with weight gain. Among the overweight, low baseline fitness and an increase in physical activity were the factors most consistently associated with weight loss. CONCLUSION: Weight loss in young adults varies by race, sex and body weight and is associated with a variety of behaviors and attitudes, some of which are also associated with weight gain and, thus, may be associated with weight fluctuation.