The effect of season on the energy balance was examined in 114 young adult Dutch women consuming self-selected diets. Energy intakes and patterns of physical activity were assessed monthly 14 times with the 24-h recall method. After this period of 14 months, in the second year the same estimates were made with intervals of 2-3 months to check if the seasonal variations observed were not accidental. The study did not demonstrate seasonal variations in the mean energy intake of the group under study. However, the intake of fat appeared to be higher in the winter and spring than in the summer and autumn, whereas for the intake of mono- and di-saccharides the reverse seemed to be true. The intake of dietary fibre was higher in the autumn than in the summer, with intermediate values for winter and spring. Small seasonal fluctuations were observed in body weight and time spent on various physical activities. On the one hand, these fluctuations are too small to indicate physiological significance, on the other hand they are wide enough to be taken into account in the design of many longitudinal studies on the relation between diet and disease.