The occurrence of different porphobilinogen deaminase mutant types in 68 patients with acute intermittent porphyria from 33 unrelated families in Finland was studied with biochemical and immunological techniques. In this fairly homogenous population four different porphobilinogen deaminase mutant types were identified and their frequencies determined. Most (about 80%) of the mutations were cross reacting immunological material (CRIM) negative, including a large kindred with normal erythrocyte porphobilinogen deaminase activities. The remainder of the families had CRIM positive mutations, including an unusual type (type 2) that had an immunoreactive, non-catalytic porphobilinogen deaminase level considerably greater than the maximal theoretical ratio of CRIM to activity of 2.0 for a single mutant allele. Correlations of the amount of residual porphobilinogen deaminase activity and the occurrence of acute clinical manifestations in each mutant type suggested that CRIM positive type 2 patients may have fewer acute symptoms.