Acute intermittent porphyria (AIP) is due to a defect in porphobilinogen deaminase (PBGD, E.C. 4.1.3.8) inherited as an autosomal dominant trait. Presymptomatic carrier detection is important in order to avoid exposure to factors inducing severe clinical symptoms. Carriers and noncarriers of the AIP gene can be distinguished by linkage analysis using three intragenic RFLPs in AIP families. In the present study, the polymerase chain reaction (PCR) was used to amplify 3.3-kb genomic sequences covering three polymorphic sites. Haplotypes were identified after cleavage of amplified products with three restriction enzymes, showing that the technique can be successfully used for linkage analysis in AIP families.