Acute intermittent porphyria (AIP) is a low-penetrant autosomal dominant disorder caused by mutations in the hydroxymethylbilane synthase (HMBS) gene. Direct detection of mutations is becoming the method of choice for the accurate identification of asymptomatic affected individuals within AIP families so that they can be advised to avoid drugs and other compounds that provoke the life-threatening acute neurovisceral crises that characterise the condition. We describe a prospective comparison of direct automated sequencing of cDNA (29 patients) or genomic DNA (28 patients) to identify HMBS mutations in 57 patients referred consecutively for mutational analysis; 39 different mutations were identified in 54 patients. The sensitivity of the cDNA and genomic DNA methods was 69% and 95%, respectively, indicating that analysis of genomic DNA provides a higher mutation detection rate. Thirty mutations were restricted to a single family; only one (R173W) occurred in more than three families. Of the mutations (6 missense, 8 splice defects, 10 frameshifts, 1 nonsense), 25 have not been reported previously. One novel mutation (344+33G-->T) was located in a putative intron splice enhancer in intron7. Our results define the extent of allelic heterogeneity and the types (41% missense; 59% truncating) and distribution (35% in exons 10, 12, 14) of HMBS mutations, for AIP in the United Kingdom.