The tryptophan hydroxylase (TPH) gene encodes for the rate-limiting enzyme of the serotonin metabolism and, therefore, has to be considered a major candidate for association studies in affective disorders. Recently, an association between this gene and bipolar affective disorder has been reported in a French population. We sought to replicate this finding in a German sample. Allele frequencies of a biallelic polymorphism (A218C) of the TPH gene were determined in 95 bipolar I patients and their parents. Preferential transmission of alleles from heterozygous parents to bipolar offspring was tested with the "transmission disequilibrium test" (TDT), which eliminates the contribution of population stratification to an association finding. Our sample yielded a power >90% to detect the originally reported effect. Neither allele 218A nor allele 218C were preferentially transmitted from heterozygous parents to bipolar offspring. Our results, therefore, do not support the hypothesis that the TPH gene is involved in the etiology of bipolar disorder.