Serum CA19-9 levels were measured in 60 diabetic patients and 40 healthy volunteers. Serum CA19-9 concentration was correlated with hemoglobin A1 (HbA1) (r = 0.4368 P less than 0.005) and fasting plasma glucose levels (r = 0.3410 P less than 0.01). None of the 40 healthy subjects showed elevated CA19-9 concentrations over 37 units/ml as the upper normal value. The percentage of positive serum CA19-9 levels in poorly controlled patients (fasting plasma glucose greater than 200 mg/dl or HbA1 greater than 13%) and moderately to well controlled patients was 50% and 10%, respectively. No correlation was found between the level of CA19-9 and those total cholesterol, and triglycerides, or the duration of diabetes. In patients who had diabetic retinopathy or persistent proteinuria, the CA19-9 concentration was significantly elevated when compared with those without these complications. It has been shown that patients with adenocarcinoma of the gastrointestinal tract have high plasma CA19-9 levels and those who have benign disease have normal CA19-9 levels. Even though diabetes mellitus is not a malignant disease, serum CA19-9 levels were increased in diabetic patients. These results indicate that HbA1 and fasting plasma glucose should be examined in patients with high CA19-9 levels.