The course of acute intermittent porphyria is described in a patient who died during an acute exacerbation of the disease. An analysis of the urinary porphyrin precursors (delta-aminolaevulinic acid and porphobilinogen), the determination of toal porphyrin excretion and the separation of haem precursors in the urine according to the number of carboxylic groups demonstrate different degrees of biochemical severity in the individual consanuinious members of this patient's family. The detection of latent carriers is of particular importance since the avoidance of porphyrogenic substances is the most important prophylactic measure to be undertaken in all latent clinical cases. According to our experience, however, prophylactic measures must also be extended to young consanguineous family members with negative excretion patterns in view of the difference in age at which the disease manifests itself.