In order to detect deletion mutation and/or gene conversion in the 21‐hydroxylase (21‐OH) gene, we adopted the polymerase chain reaction (PCR) method followed by electrophoresis. Two pairs of synthesized primers, Ta/lb and 2a/2b, each corresponding to the sequence at the 5′ portion of the 21‐OH gene, were set for PCR.Taql digestion of amplified DNA from normal individuals using Ta/lb as primers gave the following three fragments: an active 21‐OH gene‐derived 559 bp fragment, and pseudogene‐derived 364 and 195 bp fragments. Of 16 patients with 21‐hydroxylase deficiency (21‐OHD) studied, 6 (37%) lacked the 559 bp fragment. These 6 patients also lacked both the 331 and 117 bp Mval fragments of the PCR product which were obtained with the primers 2a/2b, both being derived from the active 21‐OH gene. These results indicate that 6 of the 16 patients have either deletion of the 21‐OH gene or conversion of the gene to its tandemly located pseudogene. The method described here provides a rapi