We studied the configuration of the complement C4/CYP21 (steroid 21-hydroxylase) region of the human major histocompatibility complex in patients suffering from congenital adrenal hyperplasia (CAH) and in the general population in The Netherlands, using C4 and CYP21 probes and the restriction enzymes TaqI and Bg/II. We found a rare TaqI 3.9-kb restriction fragment in the mother of a CAH patient, and present evidence that this polymorphism is caused by an additional restriction site in the first intron of a complement C4 gene.