Data available from in-vitro and in-vivo studies suggest oncostatic properties of peroral antidiabetics, thiazolidinediones, in many types of cancer. This study is the first report on the chemopreventive effect of pioglitazone in mammary carcinogenesis in rats. Mammary carcinogenesis was induced by N-methyl-N-nitrosourea administered in two intraperitoneal doses per 50 mg/kg bodyweight on the 43rd and 50th postnatal days. Pioglitazone was administered in the diet at concentrations of 10 and 100 ppm, respectively, 12 days before the first carcinogen dose until the termination of the experiment. During the experiment, the animals were weighed weekly and palpated for the presence of mammary tumors, and the incidence, latency, tumor frequency, and tumor volume were recorded. The experiment was terminated 17 weeks after the first carcinogen dose; basic tumor growth parameters and metabolic and hormonal variables were evaluated. Pioglitazone at higher concentration decreased incidence and frequency per group from the 11th week of experiment when compared with the control group and a group receiving a lower dose. Pioglitazone at a higher dose decreased the final incidence by 38%, frequency per group by 63%, and extended latency period by 32% when compared with the control group. Our data suggest that pioglitazone and other glitazones should be further investigated for oncopreventive effects.