Recent data from several reports indicate that free radicals are involved in the biochemical mechanisms underlying bipolar mood disorders (BMD) in humans. There has been increasing evidence that deranged catalase (CAT) activities might be a risk factor for BMD. Polymorphisms in the genes encoding antioxidant enzymes should therefore result in predisposition to BMD. In this study we examined the distribution of a functional polymorphism, −262C→T, in the gene for catalase, in an independent population of 96 BMD patients and 93 healthy individuals. The presence of the polymorphism, which results in the elimination of a SmaI restriction site, was tested with a PCR amplification/SmaI digestion-based assay. No significant difference has emerged from the comparison of either genotype or allele frequencies (P<0,05). We conclude that the catalase gene −262 C→T polymorphism does not confer a protective effect with respect to BMD.