We performed a family study to investigate the heritability of reduced serum retinol levels observed in type 1 diabetes cases. Diet and serum factors, including retinol, total carotene, malondialdehyde, and retinol binding protein levels, were measured in 11 multiple-case families. The mean serum retinol level of the diabetics (46 ug/dl) was significantly less than the mean serum retinol level of the nondiabetics (60.9 ug/dl). The level of retinol binding protein was also significantly lower in diabetics (6.2 mg/dl) than in nondiabetics (7.6 mg/dl). The serum values of retinol binding protein were closely related within families, including both diabetic and nondiabetic family members. A characteristic shared between diabetics and one-third of their family members was a low ratio of serum retinol to total carotene, suggesting a low conversion of dietary carotene into retinol. Analysis of food frequency reports showed no difference between dietary retinol or total carotene level in diabetics or their relatives. This study offers evidence that heritability and the reduced conversion of carotene may play a role in the level of serum retinol in type 1 diabetes cases.