The diagnosis of acute intermittent porphyria was made in 10 members of a large kindred because of increased excretion of porphobilinogen and delta-aminolevulinic acid in the urine but normal fecal porphyrins. Erythrocyte uroporphyrinogen I synthase activity was normal in all nine subjects in whom it was measured. No hematologic or other cause was found that could secondarily have raised low activity to normal, suggesting that the porphyric subjects may have had no enzyme abnormality in their erythrocytes. On the other hand, in 49 other Finnish patients with acute intermittent porphyria who were unrelated to he kindred, erythrocyte uroporphyrinogen I synthase activity was low. In acute intermittent porphyria there may be two variants, in one of which the enzyme defect is not expressed in the erythrocytes.