17alpha-Hydroxylase deficiency is a rare disease caused by mutation of the CYP17 gene, resulting in hypertension, hypokalemia, female sexual infantilism or male pseudohermaphroditism, low blood cortisol and low plasma renin activity. Herein, we report a female Taiwanese with 17alpha-hydroxylase deficiency. The CYP17 genes of this patient and five members of her family were analyzed by PCR-direct sequencing. One allele of the patient contains a 9-bp (c. 1459-1467 GACTCTTTC: D487, S488, F489) deletion, which is prevalent in Southeast Asia. The other allele has a 6-bp (c. 1480-1485 AAGGTG: K494, V495) deletion and an R496L (c. 1487 G>T) missense mutation, which is a novel mutation. Site-directed mutagenesis, in vitro expression and functional analysis in HEK-293T cells showed that this novel mutation [K494_V495 Del; R496L] resulted in complete loss of 17alpha-hydroxylase and 17,20-lyase activity. Thus this novel mutation in the extreme C-terminus abolishes enzyme activity, and when accompanied by a 9-bp deletion at codons 487-489 in the other allele, results in 17alpha-hydroxylase/17,20-lyase deficiency in this patient.