Heme biosynthesis was examined in erythroid tissue of a 4-yr-old girl with severe sideroblastic anemia since infancy, as documented by the presence of intramitochondrial deposits of iron in erythroblasts. Free red cell protoporphyrin, urinary porphyrins, and activities of erythrocyte porphobilinogen synthase, uroporphyrinogen 1 synthase, aspartate aminotransferase, and pyridoxine kinase were normal or increased. Bone marrow ferrochelatase activity was normal. Activity of bone marrow delta-aminolaevulinate (ALA) synthase was markedly reduced to 7 pmole ALA/10(6) erythroblasts/30 min (normal 127 +/- 29) but was enhanced fivefold by pyridoxal phosphate (normal 0%--25% increase). Therapy with oral pyridoxine and parenteral pyridoxal-5′-phosphate did not increase effective red cell production. The sideroblastic anemia in this patient appears to be related to a congenital defect in the initial step of heme biosynthesis.