OBJECTIVE: The impact of season on energy expenditure and physical activity is not well quantified. This study focused on summer-winter differences in total energy expenditure (TEE) and physical activity. RESEARCH METHODS AND PROCEDURES: Twenty-five healthy Dutch young adults, living in an urban environment, were measured in the summer season and the winter season. TEE was measured using doubly labeled water, and sleeping metabolic rate (SMR) was measured during an overnight stay in a respiration chamber. Subsequently, the physical activity level (PAL = TEE/SMR) and activity-related energy expenditure [(0.9 x TEE) - SMR] were calculated. Maximal mechanical power (Wmax) was determined with an incremental test on a cycle ergometer. Body composition was measured with hydrostatic weighing and deuterium dilution using Siri's three-compartment model. RESULTS: There was no difference in TEE between seasons. PAL was higher in summer than in winter (1.87 +/- 0.22 vs. 1.76 +/- 0.18; p < 0.001), and the difference was higher for men than for women (0.20 +/- 0.14 vs. 0.05 +/- 0.16; p = 0.04). The difference in PAL between seasons was dependent on the initial activity level. There was a strong linear relation (R2 = 0.48) between PAL and physical fitness (Wmax/fat-free mass), but Wmax/fat-free mass did not change between seasons in response to the lower PAL in winter. DISCUSSION: The extent of the changes in PAL is of physiological significance, and seasonality in physical activity should be taken into account when studying physical activity patterns or relationships between physical activity and health.