The second enzyme of the porphyrin and heme synthesis pathway, porphobilinogen synthase (EC 4.2.1.24), can be lowered due to a hereditary defect or toxic damage. Lead is the most common cause for a toxically lowered activity. We report on a 48-year-old patient with an acute abdominal syndrome, anemia and a grave toxic disorder of porphyrin metabolism which was diagnosed as acute lead intoxication. Although the hematologic findings and porphyrins in the blood, as well as porphyrin precursors and porphyrin excretion in the urine, returned to normal within five months after exacerbation of the acute symptoms the activity of porphobilinogen synthase showed only 50% physiologic activity of healthy subjects six years after the acute syndrome with complete normalization of prophyrin metabolites and lead levels in blood and urine. The influence of alcohol was excluded. Enzyme activation by zinc and reactivation by dithiothreitol lies within the normal range and thus contradicts an increased body burden of lead. In connection with findings of a familiar porphobilinogen synthase defect we are most likely dealing with a hereditarily determined enzyme deficiency in this patient which sensitizes him to a lead intoxication.