Fatty acid amide hydrolase (FAAH) inactivates the endogenous cannabinoid (endocannabinoid) anandamide and related lipid transmitters in vivo . A single nucleotide polymorphism (SNP) in the human FAAH gene (385C to A) has recently been described that, in homozygous form, is over-represented in subjects with problem drug use. This SNP, which converts a conserved proline residue in FAAH to threonine (P129T), suggests a potential role for the FAAH–endocannabinoid system in regulating addictive behavior. Nonetheless, the impact of the 385A mutation on the biochemical and cellular function of FAAH remains unknown. Here, we report that T-lymphocytes isolated from patients homozygous for the P129T-FAAH variant express less than half of the FAAH protein and activity observed in wild-type (WT) lymphocytes. Transfected COS-7 cells also expressed significantly lower levels of P129T-FAAH compared with WT-FAAH, indicating that the aberrant expression of the former protein is not a cell type-specific phenomenon. A comparison of the transcription/translation efficiencies and cellular stabilities of WT- and P129T-FAAH proteins revealed that the reduced expression of the mutant enzyme is due to a post-translational mechanism that precedes productive folding. These findings indicate that the natural 385A SNP in the human FAAH gene produces a mutant enzyme with reduced cellular stability, thus fortifying a potential link between functional abnormalities in the endocannabinoid system and drug abuse and dependence.