DNA was analysed from 20 patients with congenital adrenal hyperplasia due to cytochrome P-450 steroid 21-hydroxylase deficiency. Using probes recognising sequences in both the 21-hydroxylase gene and the adjacent fourth component of complement (C4), one patient was found to have a homozygous deletion of DNA which encompassed the C4B and 21-hydroxylase B genes. Evidence is presented for this deletion arising by recombination between homologous regions of 21-hydroxylase A and B. Seven patients appeared to be heterozygous for the same deletion, but no detectable alteration in the 21-hydroxylase gene could be demonstrated in others.