Milk lipids from six mothers with cystic fibrosis were compared with milk lipids from six mothers without cystic fibrosis. Mean neutral lipids (in g/dL) were colostrum, 2.4; transitional, 2.9; mature milk, 2.9, suggesting that milk fat content was sufficient to support energy needs of the infant. Lipid class composition was normal, 98% being present as triglyceride. Cholesterol and cholesteryl ester concentrations were similar in the milk of both groups of lactating women. Phospholipid class distribution of cystic fibrosis milk was different from that in control milk. Approximately 85% of the lipids were present in six major fatty acids from 12:0 to 18:2 in both groups, indicating that the profile of most of the lipid was similar. The linoleic acid content of cystic fibrosis milk was decreased significantly to 75% of normal values. Other polyunsaturated fatty acids were elevated in cystic fibrosis milk. Concentrations of 16:2, 18:3, and several longer chain polyunsaturated fatty acids were twice that in the milk of the control women, suggesting increased delta 6-desaturation. Elevated proportions of several other polyunsaturated fatty acid metabolites suggested increased chain elongation to C20 and C22 polyunsaturated fatty acids. These differences in fatty acid patterns in cystic fibrosis milk were similar to differences found in blood lipid patterns of children with cystic fibrosis. These results suggest abnormal polyunsaturated fatty acid nutrition or metabolism in cystic fibrosis.