Single-strand conformation polymorphism analysis was used to screen all 15 exons of the porphobilinogen deaminase gene from 13 patients with acute intermittent porphyria. Unique banding patterns in two amplified gene fragments, one containing exon 9 and another containing exon 10, were further investigated. Sequence analysis of cloned genomic DNA revealed a single base pair insertion in the middle of exon 9 in one patient and a single base pair deletion near the 3' end of exon 10 in two related patients. Both mutations change the reading frame of the mRNA transcript and predict proteins that are normal at their NH2-terminal ends but contain novel, unrelated sequences at their COOH-terminal ends and are prematurely terminated. Frameshift mutations in the porphobilinogen deaminase gene are uncommon; this is the first report of an insertion mutation causing acute intermittent porphyria.