A new CYP17 gene abnormality was found in three Japanese patients with 17 alpha-hydroxylase deficiency (170HD). These patients were children from consanguineous marriages, but from two apparently unrelated families: one patient with 46, XY karyotype, and two siblings with 46, XX and 46, XY karyotypes. They were all raised as girls and presented with amenorrhea, eunuchoid appearance and hypertension. Gene analysis revealed two base-pair (TG) deletion in exon 5 (codons 300, 301) of the CYP17 gene. This deletion could be expected to alter the reading frame resulting in the lack of a haem-binding region (Cys 442) due to a premature stop codon at position 333. This small mutation may account for the patients' clinical manifestations of 170HD.