To evaluate preventive effects of pioglitazone on pancreatic -cell damage in C57BL/KsJ db/db mice, an obese diabetic animal model, the pancreatic islets were compared morphologically between pioglitazone-treated (100 mg/kg daily p.o.) and untreated mice (n=7 for each) after 12 weeks' intervention (6 - 18 weeks of age). The fasting blood glucose level was significantly improved by the treatment with pioglitazone (260 ± 12 vs 554 ± 62 mg/dl, p<0.05). The islet mass in the pancreas was significantly greater in pioglitazone-treated db/db mice than in the untreated control db/db mice (10.2 ± 1.1 vs 4.6 ± 0.2 mg, p<0.01). The ratio of -cells to total cells in the islet was also greater in pioglitazone-treated mice compared to untreated mice (80.6 ± 12.0 vs 73.4 ± 2.2 %, p<0.01). No significant effects of pioglitazone on the pancreatic islet morphology were found in lean non diabetic db/+ mice. In addition, biochemical and physiological analyses of the -cell function were employed using pioglitazone-treated and untreated db/db mice (n=6 for each), and pioglitazone-treated and untreated db/+ mice (n = 6 for each). After 2 weeks' treatment (10 - 12 weeks of age), the plasma levels of triglyceride, and free fatty acid were significantly decreased, while the plasma adiponectin level increased significantly compared to untreated group (65.2 ± 18.0 vs 18.3 ± 1.3 µg/ml, p<0.05). Pioglitazone increased insulin sensitivity assessed by insulin tolerance test. Pioglitazone significantly reduced the triglyceride content in the islets (43.3 ± 3.6 vs 65.6 ± 7.6 ng/islet, p<0.05) with improved glucose-stimulated insulin secretion. Pioglitazone showed no significant effects on the biochemical and physiological parameters in db/+ mice. The present study first demonstrated that pioglitazone prevents the -cell damage in an early stage of the disease progression in db/db mice morphologically and physiologically. Our results suggest that pioglitazone improves the glucolipotoxicity by increasing insulin sensitivity and reducing fat accumulation in the pancreatic islets.