Elevation of serum alkaline phosphatase concentration in patients with diabetes mellitus has been observed for several years, but the source and reasons are unknown. We report our experience with 39 diabetics, 38% of whom had an unexplained elevation of serum alkaline phosphatase. Isoenzyme determinations revealed bone fraction as the predominant species. Mean fasting serum glucose was significantly higher in the group with elevated alkaline phosphatase, supporting an association between the severity of diabetes and diabetic bone disease.