Objective Despite the latest discovery of obesity-associated genes, the rapid rise in global obesity suggests a major role for environmental factors. This study investigated the influence of environmental factors on physical activity and dietary intake independent of genetic effects. Methods Sixteen female monozygotic twins aged 48.8 ± 9.8 years (range 37-70) with a mean BMI discordance of 3.96 ± 2.1 kg/m2 (range 0.7-8.2) were studied. Physical activity was determined using 7-day accelerometry and dietary intake using 3-day 24-h recalls. Results Heavier cotwins were generally less physically active (mean activity counts × 1,000 per day ± SD; 505.5 ± 155.1 vs. 579.6 ± 185.4, P = 0.047) and tended to spend 6.1 min/day less in moderate to vigorous physical activity than leaner cotwins (P = 0.09). Energy intake did not significantly differ within pairs. Total fat intake (en%; P = 0.03), specifically monounsaturated fat (P < 0.01) and polyunsaturated fat (P = 0.08), was higher in the heavier cotwins. Conclusions After eliminating genetic effects, higher BMI is associated with lower overall and moderate to vigorous physical activity and higher intake of total fat, although the direction of causality cannot be determined. Future identification of the environmental factors responsible for these findings might contribute to developing new strategies in managing obesity.