Tyrosine hydroxylase (TH) is the ratelimiting enzyme in the synthesis of dopamine and norepinephrine. It may be involved in the pathophysiology of psychiatric disorders and positive associations have been reported for TH gene markers in mood disorders. While most replications failed to confirm the initial findings, other papers suggested a potential role of this gene in the etiology of mood disorders. Among the many different reasons for a lack of consistent replications, a critical role is played by the ''correct'' phenotype identification. Actually, up to now the only classification criteria has been the psychiatric diagnosis, but within the same psychiatric diagnoses the symptomatologic presentation may vary dramatically depending upon severity, presence of psychotic features or other psychopathologic traits. Thus, the aim of our study is to evaluate a possible association for TH gene with symptomatology in a sample of subjects affected by mood disorders. We have developed a phenotype definition based on the observed symptomatology divided into the four factors ''Excitement,'' ''Depression,'' ''Delusion,'' and ''Disorganization.'' Our sample includes 46 mood disorder subjects, investigated by the OPCRIT (operational criteria checklist for psychotic illness) checklist for their symptomatological pattern and typed for TH variants by polymerase chain reaction (PCR) amplification. Depressive factor was associated with TH variants (F = 4.79, df = 4, 87, P = 0.006), with TH*2 subjects presenting lower depressive scores. Subjects with genotype TH*2/2 were the only ones in the sample to report mild depressive episodes. TH variants may be re-lated with depressive symptomatology in subjects affected by mood disorders. Am. J.