AbstractTryptophan hydroxylase (TPH) is the rate‐limiting enzyme in the biosynthesis of serotonin, and association and linkage studies of its variants in suicidal and impulsive‐aggressive behavior have brought conflicting results. This pilot study was designed to investigate whether TPH A218C genotypes could be associated with impulsive behavioral tendencies (IBTs) in consecutively admitted nonpsychotic nonorganic inpatients. Patients (20 females and 34 males; age, 38.8 ± 11.8) did not differ from healthy nonimpulsive controls (16 females and 11 males; age, 35.2 ± 10.2) regarding TPH genotypes, but in the patients, the number of IBT was related to the presence of the 218C allele. It was concluded that impulsive‐aggressive behavior may be associated with the TPH genotype in well‐characterized impulsive patients and that the present results stress the importance of considering impulsiveness‐aggressiveness in studies investigating the relationship between suicidal behavior and TPH genotypes. © 2002 Wiley‐Liss, Inc.