We measured the serum-stimulated lipase activity, fatty acid content, and various biochemical parameters in the breast milk of a lactating mother suffering from familial lipoprotein lipase deficiency and of healthy control subjects. Serum-stimulated lipase activity was virtually undetectable in milk from our patient and the total fatty acid content was low. The fatty acid composition differed from normal showing a marked absolute and relative increase in the content of lauric (C12:0) and myristic (C14:0) fatty acids and considerably reduced levels of oleic (C18:1) and especially linoleic (C18:2) acids. Other fatty acid species showed lesser changes in concentration. Besides a reduced calcium concentration in the milk from our patient, other biochemical parameters were not significantly different from control subjects.