Changes in total lipid, fatty acids, total cholesterol, free cholesterol, and lipid phosphorus in mature milk with time were investigated. Milk samples were collected from 10 mothers at 2, 6, 12, and 16 weeks postpartum. During 1 day, each mother donated two complete breast expressions. Expressions were taken using an electric breast pump 1 h after the previous a.m. and p.m. nursing. For each mother, the a.m. and p.m. samples were pooled for analysis. It was observed that the amount of total lipid increased significantly (p less than 0.05) from 3.9 g/100 ml at 2 weeks to 5.2 g/100 at 16 weeks postpartum. The total fatty acid composition remained uniform during the investigation. Average total cholesterol and free cholesterol in the milk were 10.3 mg/100 ml and 8.3 mg/100 ml, respectively. These concentrations did not change significantly with time postpartum. Average lipid phosphorus was 3.9 mg/100 ml and also remained constant throughout. We conclude that the fatty acid pattern, lipid phosphorus, total cholesterol, and free cholesterol of mature milk to 16 weeks postpartum remains relatively constant while total lipid concentration increases.