In order to assess the beneficial effect of the peroxisome proliferator-activated receptor-γ (PPAR-γ) agonist pioglitazone on reduction of mass and alteration of function of pancreatic β cells under diabetic conditions, diabetic C57BL/KsJ db/db mice were treated with pioglitazone for 6 weeks, and insulin secretory capacity and insulin content of isolated pancreatic islets were evaluated. In addition, the expression of oxidative stress markers, 4-hydroxy-2-nonenal (HNE)-modified proteins and heme oxygenase-1, in endocrine pancreas was examined to measure reduction of oxidative stress in pancreatic β cells. The capacity for glucose-induced insulin secretion from isolated islets and their insulin content were improved by pioglitazone treatment (P < .01). When β cells were stained with anti-insulin antibodies, those of db/db mice treated with pioglitazone exhibited strong staining, as also observed in their lean littermates. The density of immunostaining for oxidative stress markers was significantly reduced in pancreatic islets of pioglitazone-treated db/db mice (P < .05). This study clearly demonstrates the benefit of long-term treatment with pioglitazone in decreasing hyperglycemia and improving glucose-induced insulin secretory capacity in diabetic db/db mice. The results of immunocytochemical examination suggest that this treatment reduces oxidative stress and thereby preserves β-cell mass. Treatment with pioglitazone thus protects against β-cell damage and would be useful for restoration of insulin secretory capacity in obese diabetes individuals.