Seasonal variations of blood lipids, which must be considered when performing long-term studies, could be partially due to dietary changes. In the present study, serum lipid parameters were measured each month for 1 year in nuns living in a monastery, whose diet was perfectly regular and controlled. The serum lipid variations observed consisted mainly of an increase in total cholesterol, low-density lipoproteins and apolipoprotein B in autumn and spring and an increase in high-density lipoprotein cholesterol in winter, whereas apolipoproteins A-I and A-II showed a marked increase in summer and at the beginning of autumn. These variations were probably induced by factors directly related to seasonal rhythm and not to dietary factors or physical exercise. Lastly, the influence of sex hormones seems to be very minor, as postmenopausal women (one-third of the persons) had an identical variation in their lipid parameters, to that observed in the whole group.