Serum CA19-9 levels are often elevated in diabetic patients. To elucidate this mechanism, we investigated the metabolism of CA19-9 in diabetic patients without obvious cancer. Study 1 included 119 patients in whom HbA1c, glycated albumin (GA) and CA19-9 were measured at the time of hospital admission. Study 2 examined 6 patients with markedly elevated CA19-9 levels (≥100 U/mL). Their half-lives for HbA1c, GA, and serum CA19-9 were calculated using the data before and after diabetes treatment. Three diabetic patients with pancreatic cancer were also examined as controls. In Study 1, serum CA19-9 (logarithmically transformed value) was significantly correlated with fasting plasma glucose (FPG), HbA1c and GA. On multivariate analysis, GA and FPG, but not HbA1c, were significant explanatory variables for serum CA19-9. In Study 2, serum CA19-9 decreased together with HbA1c and GA after diabetes treatment. The calculated half-lives for HbA1c, GA, and serum CA19-9 were 33.8 days, 16.1 days, and 10.9 days, respectively. The half-life of serum CA19-9 was longer in the study patients than that reported in patients with malignant tumors. By contrast, in the diabetic patients with pancreatic cancer serum CA19-9 showed a marginal decrease after diabetes treatment. Taken all together, prolonged half-life of serum CA19-9 may contribute to the increase in serum CA19-9 levels in diabetic patients without obvious cancer.