The increased prevalence of overweight adults has serious health consequences. Epidemiological studies suggest an association between low activity and being overweight, however, few studies have objectively measured activity during a period of weight gain so it is unknown whether low activity is a cause or consequence of being overweight. To determine whether individual differences in adult weight gain are linked to an individual's activity level we measured activity, via accelerometry over a pro,longed period (9 months) in 18 adult female rhesus monkeys. Weight, food intake, metabolic rate and activity were first monitored over a 3-month period. During this period, there was mild but significant weight gain (5.5 ± 0.88%; t =-6.3, df=17, p<0.0001), while caloric intake and activity remained stable. Metabolic rate increased, as expected, with weight gain. Activity level correlated with weight gain (r=-0.52, p=0.04) and the most active monkeys gained less weight than the least active monkeys (t =-2.74, df=8, p=0.03). Moreover, there was an 8-fold difference in activity between the most and least active monkeys, and initial activity of each monkey was highly correlated with their activity after 9 months (r=0.85, p<0.0001). In contrast, food intake did not correlate with weight gain and there was no difference in weight gain between the monkeys with the highest versus the lowest caloric intake, total metabolic rate, or basal metabolic rate. We conclude that physical activity is a particularly important factor contributing to weight change in adulthood and that there are large, but stable, differences in physical activity among individuals.