A 41-year old male with insulin-dependent diabetes mellitus previously unsuccessfully treated with a controlled diet and glibenclamide, and subsequently with increasing insulin doses (5 and 20 IU/day) experienced polyuria, glycosuria and loss of weight. On admittance to hospital serum C3 concentrations were found to be depressed. The insulin dose was further increased to 30 IU/day and the patient was also treated with 20 mg nafamostat mesylate given intravenously twice daily for 6 days. On completion of nafamostat mesylate treatment serum C3 concentrations were increased but after 17 days they started to decrease again.