In vitro studies in bacterial, yeast and eukaryotic systems have demonstrated the existence of deletion and insertion 'hot-spots' involving repetitive sequences. Slipped-strand mispairing (SSM) has been suggested to be the mechanism involved. Progress in human molecular genetics has allowed the identification of many mutations causing diseases. Analysis of sequences involved in these mutations provides an opportunity to investigate the contribution of short tandem repeats to the naturally occurring mutations in coding regions of human genes. We have analyzed the sequences surrounding 625 disease-causing mutations in the coding regions of three genes: the cystic fibrosis transmembrane conductance regulator, beta globin and factor IX. Altogether, 134 (21%) insertion and deletion mutations of 4 base pairs or less were identified. In 47% of these mutations, the deletions and insertions occurred within a unit repeated tandemly 2- to 7-fold. These were classified as SSM mutations. The proportion of SSM mutations was significantly higher than expected by chance. The estimated net proportion of deletion and insertion mutations attributed to SSM was 27%. These results indicate that very short repetitive sequences contribute significantly to the generation of deletion and insertion mutations in human genes, and to the evolution of diversity of their coding regions.