The aim : Pancreatic cancer, a rapidly progressive malignancy, is often diagnosed in patients with diabetes. The incidence of pancreatic cancer has risen dramatically over recent decades. Early diagnosis of this malignancy is generally difficult because the symptoms do not become apparent until the disease has progressed, generally leading to a poor outcome. To achieve earlier diagnosis, we analyzed the clinical characteristics of pancreatic cancer patients showing deterioration of plasma glucose levels while hospitalized. Method : Thirty-six cases were divided into 2 groups ; those diagnosed with diabetes more than a year prior to identification of pancreatic cancer and diabetes secondary to pancreatic cancer. These 2 groups were further subdivided according to the tumor site (head or body / tail), allowing analysis of 4 subgroups. Anthropometric measurements, laboratory values were determined. Results : Both groups with diabetes lost at least 4 kg and showed HbA1c deterioration of at least 1% within 5 months of the pancreatic cancer diagnosis. The post-meal elevation of serum C-peptide immunoreactivity (CPR) was significantly decreased in the group with cancer of the pancreatic head, and this was unrelated to tumor size. Conclusion : Characteristically, pancreatic head cancer was associated with decreased endogenous insulin secretion as compared to body / tail cancer