Research investigating fiber intake and changes in weight over time has not controlled for important covariates, especially physical activity. Moreover, studies have rarely examined the influence of fiber on changes in body fat, only weight. Hence, this study was conducted to determine whether changes in fiber intake (total, soluble, and insoluble) influence risk of gaining weight and body fat over time. Another objective was to examine the influence of age, energy intake, activity, season, and other potential confounders. A prospective cohort design was used and 252 women completed baseline and follow-up assessments 20 mo apart. Diet was measured using 7-d weighed food records. Fiber was expressed per 1000 kcal (4187 kJ). Body fat was assessed via the Bod Pod and physical activity was measured using accelerometers over 7 consecutive days. Across the 20 mo, almost 50% of the women gained weight and fat. For each 1 g increase in total fiber consumed, weight decreased by 0.25 kg (P = 0.0061) and fat decreased by 0.25 percentage point (P = 0.0052). Controlling for potential confounders did not affect the relationships, except changes in energy intake, which weakened the associations by 24-32%. Soluble and insoluble fibers were borderline predictors of changes in weight and fat. In conclusion, increasing dietary fiber significantly reduces the risk of gaining weight and fat in women, independent of several potential confounders, including physical activity, dietary fat intake, and others. Fiber's influence seems to occur primarily through reducing energy intake over time.