—An increased plasma plasminogen activator inhibitor-1 (PAI-1) level is a risk factor for myocardial infarction, particularly when associated with visceral obesity. Although the link between PAI-1 and obesity is well documented, little is known about the physiological relevance of PAI-1 production by adipose tissue. Therefore, we have compared adipose tissue development and insulin resistance plasma parameters in PAI-1–deficient mice (PAI-1^−/−) and wild-type littermates (PAI-1^+/+) in a model of nutritionally induced obesity. After 17 weeks of consuming a high-fat diet (HFD), PAI-1^+/+ mice showed marked obesity, with a 52% increase in body weight compared with mice that were kept on a standard fat diet (P<0.0001). This weight gain was accompanied by adipocyte hypertrophy and an increase in the number of stroma cells in the gonadal fat pad, expressed as stroma cells/adipocytes (0.67±0.05 versus 0.43±0.02; P<0.001). In plasma, the HFD induced a marked increase in PAI-1 antigen (5.1±0.56 versus 2±0.22 ng/mL; P<0.001), fasting insulinemia (1.1±0.21 versus 0.21±0.04 ng/mL; P<0.001), and glycemia (7.4±0.5 versus 5±0.3 mmol/L; P<0.001), whereas plasma triglyceride levels were not affected. When we compared PAI-1^−/− and PAI-1^+/+ mice on the HFD, PAI-1^−/− mice gained weight faster than did PAI-1^+/+ mice, with a significant difference in body weight between 3 and 8 weeks of the diet (32±1.7 versus 26±1.6 g at 6 weeks; P<0.05). After 17 weeks of the HFD, its effect on weight gain and the number and size of adipocytes was similar in PAI-1^+/+ and PAI-1^−/− mice. By contrast, the increase in the number of stroma cells presented by PAI-1^+/+ mice was not observed in PAI-1^−/− mice. In obese PAI-1^−/− mice, tissue-type PA activity and antigen levels in the gonadal fat pad were significantly higher than in obese PAI-1^+/+ mice (230±50 versus 47±20 arbitrary units/g, P<0.01; 40±13 versus 17±13 ng/g, P<0.05, respectively), whereas urokinase-type PA activity and antigen levels were similar in both groups. In plasma, nonobese PAI-1^−/− mice displayed 62% higher insulin levels (P<0.05) than did PAI-1^+/+ mice. Obese PAI-1^−/− mice displayed 68% higher triglyceride levels (P<0.01) and 21% lower glucose levels (P<0.05) than did PAI-1^+/+ mice. These data support an effect of PAI-1 on weight gain and adipose tissue cellularity in the induction of obesity in mice. Moreover, PAI-1 influences glucidolipidic metabolism. The elevated expression of PAI-1 observed in human obesity could be involved in mechanisms that control adipose tissue development.