Lipids were analyzed in 308 human milk samples, obtained during the first 5 wk of lactation from 17 mothers delivering at term. The mean triglyceride content increased mainly during the 1st postpartum wk and remained constant thereafter. In contrast, the cholesterol concentration decreased, whereas the phospholipids remained rather constant, resulting in a decrease of both the phospholipid/triglyceride and the cholesterol/triglyceride ratio. Circadian variations of the parameters analyzed were not uniform and differed in milks obtained from German mothers as compared to milks obtained from English mothers. The fatty acid composition was determined in the total lipids and in the phospholipid fraction as well. Changes were found in progressing lactation, but not during the day. Some data are discussed with regard to their possible physiological or physiochemical relevance.