Recently, we reported that low molecular weight (LMW) chitosan (chitosan lactate, average MW: 20,000) prevents the progression of low dose (100 mg/kg, i.p.) streptozotocin-induced slowly progressive diabetes mellitus in male ICR mice. The present study was designed to clarify the effects of LMW chitosan on hyperglycemia, hyperinsulinemia and hypertriglyceridemia in genetically obese diabetic male KK-Ay mice. LMW chitosan (0.05%, 0.2% or 0.8% water solution) was given daily as drinking water to male KK-Ay mice for 11 weeks, from 5 weeks of age. The non-fasting serum glucose levels of control mice continued to increase slowly throughout the experimental period. LMW chitosan lowered the serum glucose levels in a dose-dependent manner. In these diabetic mice, hyperinsulinemia and hypertriglyceridemia were observed, and LMW chitosan was dose-dependently effective in improving both serum biochemical parameters. LMW chitosan at three doses improved overdrinking and polyuria observed in these diabetic mice. It is concluded from these results that LMW chitosan may be useful for the treatment of obesity-related type 2 diabetes mellitus.