We treated and followed up for 6 years a patient with pyridoxine-responsive sideroblastic anemia. The patient was a boy age 1 year and 9 months, who was diagnosed on the basis of peripheral red cell morphology and an increased number of sideroblasts in the bone marrow. Bone marrow erythroblasts showed a marked reduction of delta-aminolevulinic acid synthase (ALA-S) activity. The response of the patient to pyridoxine and its active form, pyridoxal phosphate, was unique. After the first course of pyridoxal phosphate therapy (300 to 500 mg/day i.v. for 4 days), all hematological data were restored to normal and remained normal for 29 months without the further administration of pyridoxal phosphate. The second course of pyridoxal phosphate therapy (500 mg/day i.v. for 2 days) was effective for 6 months. The third, fourth, and fifth courses of the therapy consisted of daily oral pyridoxine hydrochloride at a dose of 180 mg/day for 4 to 6 weeks, and the respective periods of hematological remission were 7, 12, and greater than 18 months. These observations suggest the presence of a complicated ALA-S activating or inactivating system, or both, in our patient.