Steroid 21-hydroxylase deficiency is caused by defectiveness of the CYP21 gene. Such defects have presumably originated from interactions with the nearby CYP21P pseudogene during evolution. We studied these mechanisms by comparing the genetic variability of CYP21, CYP21P, and CYP21P/CYP21 hybrids (resulting from large-scale rearrangements) at eight mutation sites in a group of Dutch steroid 21-hydroxylase deficiency patients, their family members, and controls. The most common CYP21 defect in patients with salt-losing steroid 21-hydroxylase deficiency was a splice junction mutation in intron 2. The most common defect in the simple virilising form of the disease was ile72 --> asn. CYP21P showed considerable sequence variation in its central and 3' sections; the 5' section was constant. A single nucleotide (T) insert in exon 7 was found in all CYP21P genes. During the course of evolution, this was probably the third defect introduced into CYP21P after the splice junction mutation in intron 2 and the 8 bp deletion in exon 3. Gene conversions introducing CYP21-like sequences contribute to CYP21P variability. Such an event has occurred de novo in one family. A comparison of CYP21 and CYP21P mutations on the same chromosome shows that at least some of the small-scale gene conversions that supposedly transfer defects to CYP21 involve interaction between homologous chromosomes. The majority of the putative CYP21P-CYP21 transitions in hybrid genes appears to occur in a distinct zone that lies 5' of nucleotide 2108, which is further downstream than previously hypothesised. The other transitions lie upstream of nucleotide 999. Apparent 'large-scale' CYP21-CYP21P gene conversions lead to hybrid genes that are very similar to those found in CYP21 deletions, so these haplotypes have probably resulted from a meiotic double unequal crossover.