Peroxisome proliferator-activated receptor gamma (PPARgamma) is a transcription factor implicated in adipocyte differentiation, lipid and glucose metabolism. A polymorphism corresponding to a silent C-->T substitution was detected in exon 6 of the PPAR gamma gene. We analysed the relationships between this genetic polymorphism and various markers of the obesity phenotype (body weight, body mass index, waist:hip ratio and plasma leptin levels) in a representative sample of 820 men and women living in northern France. The frequencies of the C and T alleles were 0.860 and 0.140 respectively. In the whole sample no association of the polymorphism with the markers tested was observed but a statistically significant interaction ( P < 0.03) existed between this polymorphism and body mass index for plasma leptin levels. This result suggested that the impact of the PPAR gamma gene polymorphism on plasma leptin levels differed according to the BMI of the subjects. Indeed, obese subjects (BMI >30 kg/m2) bearing at least one T allele ( CT + TT ) had higher plasma leptin levels than subjects who did not (35.0 +/- 17.4 ng/ml versus 28.3 +/- 14.8 ng/ml respectively; P < 0.001). This effect existed in both genders, despite the higher plasma leptin levels observed in women. The plasma leptin level increase was not associated with elevation of body mass index, even though these two variables were highly correlated. Thus for a given leptin level the BMI was relatively lower in obese subjects carrying at least one T allele than in obese CC homozygotes. Our results show that in obese subjects variability within the PPAR gamma gene locus is associated with circulating leptin levels and may modify the relationship between leptin levels and adipose tissue mass.