Hydroxyurea increases fetal haemoglobin in many patients with sickle cell anaemia, but its effectiveness in thalassaemia appears to be less consistent. We describe the response to hydroxyurea in an adult male with homozygous beta thalassaemia, symptomatic paraspinal extramedullary haemopoiesis, bone pain, and progressive tissue iron loading. Prior to therapy with hydroxyurea the circulating haemoglobin (Hb) concentration was 7.0 g/dl and absolute fetal haemoglobin concentration was 5.0 g/dl. Administration of sodium phenylbutyrate had induced no increase in either parameter. Subsequent therapy with hydroxyurea was associated with increases in total haemoglobin to 9.0 g/dl, and in fetal haemoglobin to 7.6 g/dl. Ineffective erythropoiesis was reduced and extramedullary haemopoiesis regressed during therapy.