Phenotypic expression of the Trp64Arg mutation in the beta 3-adrenoceptor gene (beta 3-AR) has been found to be somewhat variable among different populations, suggesting that it may be influenced by genetic background and environmental factors. As sex may also influence gene allellic expression, we evaluated a potential gender effect of the Trp64Arg mutation in 292 morbidly obese subjects [body mass index (BMI) > or = m/kg2]. Although the 15 mutated obese females were younger than the non-mutated ones, the difference between their current weight and their weight at 20 years was significantly higher (62.4 +/- 20.0 kg versus 47.0 +/- 24.0 kg; p = 0.017). Moreover, in the mutated heterozygous female group, the mean Zscore (individual BMI minus reference French population mean BMI/SD of reference population BMI) was significantly higher (8.0 +/- 2.5 versus 6.0 +/- 2.0 SD of BMI, p = 0.0018), as was the maximal Zscore calculated from the maximal BMI that obese females reached during life (9.0 +/- 3.0 versus 7.0 +/- 2.5, p = 0.005). The regression curves of the Zscore against age showed that the curve of mutated females was shifted to the top, indicating that their BMI was higher regardless of age. These effects were not observed in the male group (the Zscore was 6.7 +/- 3.0 vs. 7.2, p = 0.7 respectively in mutated and non-mutated men). These data reinforce the hypothesis that the expression of the beta 3-AR susceptibility gene depends on additional factors including gender and possibly hormonal status.