We report on two German sisters with deficiency in the 17alpha-hydroxylase/17,20-lyase enzyme corresponding to typical hormone profile. A paternal nonsense mutation R388X in exon 7 and a maternal missense mutation P428L in exon 8 of the CYP17 gene have been identified in both girls. Residual in vitro 17alpha-hydroxylase activity for the conversion of [3H]-Preg to [3H]-17OH-Preg has been detected in transfected 293-cells expressing P428L mutant enzyme; however, no 17,20-lyase activity was observed converting [3H]-17OH-Preg into [3H]-DHEA. The 46,XX-sister spontaneously entered puberty. The 46,XY-sister with a predicted adult height of 203 cm was treated with a high dose of conjugated estrogens and resulted with a final height of 186.9 cm. The present data suggest that compound heterozygous 46,XX females bearing a P428L allele may develop spontaneous onset of puberty. Furthermore, in 46,XY females with tall stature, treatment with conjugated estrogens may lead to a significant reduction of their predicted adult height.