In three patients who developed biotin deficiency during parenteral alimentation, serum fatty acid compositions of the four major lipid classes (phospholipid, cholesteryl ester, triglyceride and free fatty acid) were measured. Relative to the normal range, percent compositions of an odd-chain fatty acid (either 15:0 or 17:0 or both) were increased in each class of lipid, and these abnormalities generally returned to normal or decreased toward normal with biotin therapy. Abnormalities in particular fatty acids in the omega 6, omega 3 and omega 9 pathways were also found, but these abnormalities did not resolve with biotin therapy. These data provide evidence in favor of the conclusion that biotin deficiency causes increases in the composition of some odd-chain fatty acids in humans, perhaps by the same mechanism that leads to odd-chain fatty acidemia in the inborn deficiency of the biotin-dependent enzyme propionyl-CoA carboxylase.