Long-term variations in breast-milk fat concentration of mothers feeding on demand were studied in 120 rural West African women over a 12-month period. 2. The over-all mean 12 h breast-milk fat concentration was 39.3 g/l. 3. Mean breast-milk fat concentrations were affected by season in a manner which was correlated with seasonal changes in maternal subcutaneous fat stores (P less than 0.05) but which was unrelated to seasonal variations in maternal energy intake and breast-milk output. 4. Breast-milk fat concentrations were highest in early lactation, decreasing to a constant level during the first year. 5. There was significantly greater between-mother than within-mother variation in breast-milk fat concentrations measured in successive months, after correcting for season and stage of lactation (P less than 0.001). 6. Breast-milk fat concentrations were highest for primiparous mothers, decreasing to a constant level at parity 4 and higher. 7. A mother's relative breast-milk fat concentration was not correlated with her levels of dietary energy intake and breast-milk output but was positively correlated with her relative subcutaneous fat deposits (P less than 0.01).
