One of the remarkable features of sideroblastic anaemia is that the anaemia of patients with either the acquired or the hereditary form of the disease may respond to treatment with pyridoxine. However, in spite of the response, the characteristic morphological abnormalities remain and the doses of pyridoxine required to produce benefit are often large. The relationship between pyridoxine deficiency and the ringed sideroblast phenomenon is therefore still obscure. We have observed five cases of anaemia in patients suffering from pulmonary tuberculosis with cavitation and treated with, among other drugs, isoniazid and cycloserine or pyrazinamide, drugs known to act as pyridoxine antagonists. In these patients there was an erythroid hyperplasia and an accumulation of basophilic and poorly haemoglobinized erythroblasts. Slight megaloblastoid changes were observed in some patients. Numerous ringed sideroblasts were present. The peripheral blood showed a moderate hypochromasia with a dimorphic picture. The leucocyte and platelet counts were normal. The scrum‐iron level was normal in three patients and markedly increased in two. The anaemia and the haematological abnormalities disappeared entirely when the treatment with cycloserine or pyrazinamide was interrupted, even though isoniazid and the other antituberculous drugs were continued.