The human CYP2D6 gene codes for the enzyme, debrisoquine 4-hydroxylase, which metabolizes over 25 therapeutically important drugs. The inability to metabolize these drugs, which results in a 'poor metabolizer' (PM) phenotype, can be attributed, in some cases, to the presence of any of three previously described mutations in the CYP2D6 gene. To identify new alleles responsible for the PM phenotype, we have examined the CYP2D6 gene from individuals whose phenotypes were not consistent with their apparent genotypes. DNA sequencing revealed a single base deletion in exon 3, T1795, resulting in a frame shift and generating a stop codon one codon after the deletion. A PCR-based test was designed for this new allele (designated CYP2D6(T)) and 236 unrelated individuals from a lung cancer case control study were tested for the presence of the CYP2D6(T) mutation. Eight unrelated individuals were found to carry the D6(T) allele. Four subjects also carry the non-functional D6(B) allele and the drug metabolism phenotypes of these four D6(B)/D6(T) individuals are consistent with the D6(T) allele being responsible for reduced debrisoquine 4-hydroxylase activity. The frequency of the D6(T) allele among Caucasian controls of the case-control study was 1.8% (4/220 chromosomes).