Abstract Serum concentrations of the tumor-associated carbohydrate antigens CA19-9, CA-50, and sialyl SSEA-1 were measured in non-insulin-dependent diabetic patients without diseases causing the elevation of those antigens, and the relationship to diabetic conditions was studied. The patients of the Lewis blood group phenotype of Le a (23%) had higher serum CA19-9, CA-50, and sialyl SSEA-1 than those of Le b (67%) and Le(−) (10%). Le a patients with high HbA1c (> 10%) had significantly higher serum CA19-9 and CA-50 than those with low HbA1c (≦ 7%). Le b patients with high HbA1c also had elevated CA19-9 and sialyl SSEA-1. In Le b patients, diabetic nephropathy was associated with increased CA19-9 levels. Diabetic retinopathy was also accompanied by high carbohydrate antigens in Le b patients, but the difference was not significant. Le b patients treated with sulfonylurea or insulin had increased CA19-9 and CA-50. The changes in serum concentrations of these carbohydrate antigens might have some relationship not only to the Lewis blood phenotype, but also to diabetes.