Acute intermittent porphyria (AIP) is a metabolic disease with a variable prevalence among different countries. In some areas of southern Europe it remains to be fully evaluated. We undertook a genetic and biochemical study of 16 unrelated Spanish AIP patients and relatives. The genetic analyses showed they harboured the following mutations in the porphobilinogen deaminase gene: R173W, G111R, L278P, L238P, R116W, R26C, 340insT, 730delCT, 691del30bp, and IVS14+1g>a. The mutation R173W was found in 6 patients (37.5%), including the only patients of our series with >3 recurrent porphyria attacks. While in clinical remission, all AIP patients exhibited sustained increased excretion of porphyrins and precursors. PBG excretion showed a high between-subject variation and was not related to erythrocyte PBG deaminase activity. The study of family members allowed the identification of 22 asymptomatic AIP carriers. These included 8 persons harbouring the R173W mutation belonging to four different families. Six of these latent AIP subjects showed increased PBG elimination, and in two the urinary levels were >10-fold the normal limit. These results reinforce the hypothesis that the R173W mutation may have a high biochemical and clinical penetrance among AIP patients.