We have screened the hydroxymethylbilane synthase cDNAs of 3 patients from 2 families suffering from acute intermittent porphyria (AIP) from Scotland and South Africa using heteroduplex and chemical cleavage of mismatch analyses. Direct sequencing was used to characterise the mutations. The two novel mutations identified were a missense mutation at nucleotide position 64 in exon 3 (R22C) and a single base-pair deletion in exon 15. These mutations are predicted to affect the normal function of the enzyme and, therefore, are expected to be the primary cause of disease in these patients.