Homozygous deficiency of aminolaevulinate dehydratase (porphobilinogen synthase, EC 4.2.1.24) was diagnosed in a small child. The clinical presentation was unique since severe symptoms were already present in the neonatal period. The patient, a boy, now three-years old, had recurrent attacks of pain, vomiting, hyponatraemia and symptoms of polyneuropathy engaging motor functions including respiration. The clinical course of the disease from birth on is related, as are the results of various attempts at therapy. The patient excreted large amounts of 5-aminolaevulinic acid and coproporphyrin and minor amounts of porphobilinogen in the urine. Faecal excretion of coproporphyrin and harderoporphyrin was increased as was erythrocyte porphyrin concentration. Diagnosis was established by the finding that erythrocyte aminolaevulinate dehydratase activity was less than 5 per cent of normal in the patient and between 26 and 51 per cent of normal in both the parents, the grandfathers and a sibling. The activity of the enzyme could not be restored by the addition of dithiothreitol (10(-3) mol/l) alone, or in combination with zinc or manganese in varying concentrations. The enzyme Km did not differ between affected and nonaffected members of the family.