The massive obesity caused in rodents by the disruption of the leptin-receptor signal through genetic defects at the level of either leptin (OB) or leptin receptor (OB-R) has raised the question of the relevance of these genes to morbid obesity in humans. In this study, we screened a large population of massively obese subjects for the presence of a leptin receptor mutation homologous to that of fa/fa rats, a single base substitution changing glutamine 269, a highly conserved glutamine found at position 270 in the human sequence. After polymerase chain reaction (PCR) amplification of a DNA region encompassing the end of exon 5, intron 5, and the beginning of exon 6, we performed restriction fragment length polymorphism analysis. Within the limitations of this approach where only mutations introducing restriction sites (5 of 8 possibilities) could be assessed, no evidence of mutation at the codon gln 270 was found in 343 massively obese subjects. However, a new OB-R gene variant in intron 5 was revealed by MaeII digestion of the PCR products. MaeII/hOB-R genotyping revealed no difference in the distribution of the genotypes between obese subjects and a group of 79 unrelated nonobese control subjects. In addition, no significant association between various obesity-related metabolic phenotypes and the presence of MaeII/hOB-R alleles was found. Thus, our results did not support a significant role for the MaeII/hOB-R gene variant in the development of the obese phenotype in the population we studied.