1. Patients with insulin-dependent diabetes mellitus are classified among the groups at risk for low vitamin status, and recent studies suggest that some degree of supplementation with antioxidants may be beneficial in helping to prevent certain long-term complications of diabetes mellitus. Our objective was to compare the status of the fat-soluble vitamins and antioxidant-related compounds in patients with well-defined insulin-dependent diabetes mellitus with that of their first-degree relatives, controlling seasonal and analytical variability as factors influencing the interpretation of the data. 2. Fifty-four patients with insulin-dependent diabetes mellitus, 214 non-diabetic, first-degree relatives (controls) and 236 unrelated controls were analysed for retinol, tocopherols (alpha and gamma) and main carotenoids in serum (beta-carotene, alpha-carotene, beta-cryptoxanthin, lutein, zeaxanthin and lycopene) by means of a validated HPLC method. 3. Insulin-dependent diabetes mellitus was associated with lower retinol levels and higher levels of beta-carotene, alpha-carotene and beta-cryptoxanthin than sex-matched, first-degree relatives. alpha-Tocopherol, the alpha-tocopherol/cholesterol ratio, gamma-tocopherol, lutein, zeaxanthin and lycopene showed no differences. Retinol and beta-carotene were the variables most closely associated with diabetes. 4. Patients with insulin-dependent diabetes mellitus showed lower serum retinol status together with higher concentrations of provitamin-A carotenoids. Serum fat-soluble antioxidant levels were greater than or equal to those in controls. According to the serum status observed, individuals with diabetes do not require supplementation with alpha-tocopherol or carotenoids, although the need for retinol supplementation in patients with marginal serum levels should be evaluated.