To study prostaglandin (PG) metabolism in children with insulin-dependent diabetes mellitus, plasma levels of PGE2, PGF2 alpha, and thromboxane B2 and the composition of serum fatty acids were measured. Platelet aggregation, a risk factor in diabetic vascular complications, was also measured. The mean levels of plasma PGE2 and PGF2 alpha were high and serum dihomo-gamma-linolenic acid and arachidonic acid were low in the diabetic children. The plasma thromboxane B2 levels of the diabetic children and normal controls were not significantly different. Platelet aggregation was also increased in diabetic patients. These results suggest that the insulin deficiency and high blood sugar in diabetic children may disturb the supply of dihomo-gamma-linolenic acid from cis-linoleic acid, decreasing prostaglandin formation in series 1. Then feedback regulation may increase production of PGE2 and PGF2 alpha in series 2. Altered PG metabolism may be responsible for the occurrence and progression of vascular complications in the diabetic children.