The effect of the apolipoprotein (apo) E genotype on the lipoprotein response to a 1 year low cholesterol diet (200 mg cholesterol per day) was evaluated in 36 patients with Type 1 diabetes mellitus with albuminuria between 10 and 200 microg min(-1). Apo E genotype was characterized by polymerase chain reaction and restriction isotyping. In 11 IDDM patients with at least one epsilon4 allele (apo E4 group), baseline serum total and low density lipoprotein (LDL) cholesterol were higher (p < 0.05 for both) than in 25 patients without an epsilon4 allele and with at least one epsilon3 allele (apo E3 group). Dietary counselling resulted in a similar decrease in cholesterol intake in both groups, whereas linoleic acid did not change. In the apo E4 group, serum total and LDL cholesterol at follow-up fell (p < 0.01 for both) to levels that were not different from those in the apo E3 group, and the changes in these parameters were greater (p < 0.02) than those in the apo E3 group. We conclude that the apo E4 allele is associated with atherogenic lipoprotein abnormalities in Type 1 DM patients with minor elevations in albuminuria when they use their habitual diet. Apo E4 carrying patients respond better to a low cholesterol diet.