Comings et al. [1991: JAMA 266: 1793-1800] have recently reported a highly significant association between Tourette's syndrome (TS) and a restriction fragment length polymorphism (RFLP) of the dopamine D2 receptor gene (DRD2) locus. The A1 allele of the DRD2 Taq I RFLP was present in 45% of the Tourette patients compared with 25% of controls. We tried to replicate this finding by using the haplotype relative risk (HRR) method for association analysis. This method overcomes a major problem of conventional case-control studies, where undetected ethnic differences between patients and controls may result in a false-positive finding, by using parental alleles not inherited to the proband as control alleles. Sixty-one nuclear families encompassing an affected child and parents were typed for the DRD2 Taq I polymorphism. No significant differences in DRD2 A1 allele frequency were observed between TS probands, subpopulations of probands classified according to tic severity, or parental control alleles. Our data do not support the hypothesis that the DRD2 locus may act as a modifying gene in the expression of the disorder in TS probands.