Several candidate genes have been associated with antipsychotic-induced body weight (BW) gain. Because the endocannabinoid system is deeply involved in BW regulation, endocannabinoid genes may have a role in the antipsychotic-induced weight gain. Therefore, we investigated the 1359 G/A (rs1049353) single nucleotide polymorphisms (SNP) of the cannabinoid receptor 1 (CNR1) gene, which codes the endocannabinoid CB1 receptor, and the complementary DNA (cDNA) 385C/A (rs324420) SNP of the FAAH gene, which codes the endocannabinoid degrading enzyme, for their role in BW changes induced by antipsychotic drugs. Eighty-three white psychotic patients who underwent a naturalistic treatment with different antipsychotics (clozapine, olanzapine, risperidone, quetiapine, and haloperidol) and completed a 24-week treatment period were included into the study together with 80 age- and sex-matched white healthy controls. At the 24th week of treatment, 41 patients gained more than 7% of their baseline BW. No significant differences between patients and controls emerged in genotype and allele frequencies of both SNPs. Genotype and allele frequencies of the FAAH cDNA 385C/A SNP but not of the CNR1 1359 G/A SNP significantly differed between subjects who gained more than 7% of BW and those who did not, with both AC and AA genotypes and the A allele being significantly more frequent in patients who gained more than 7% of their baseline BW. Present findings, although obtained in a small population and in a naturalistic setting, suggest that the cDNA 385C/A SNP of the FAAH gene may predispose subjects to get a clinically meaningful weight gain after antipsychotic exposure.