Megaloblastic anaemia responding to folic acid has been reported in a number fo patients with sickle cell anaemia. The incidence of reduced serum and erythrocyte folate levels in such patients remains unclear, however. Serum and erythrocyte folate contents were measured microbiologically in patients with sickle cell anaemia as well as in two control groups with sickle cell trait or normal haemoglobin pattern. Low serum and erythrocyte folate values were significantly more common in 61 sickle-cell patients than in each of the two socio-economically-matched control groups including 61 subjects with Hb AS and 69 blacks with Hb AA. There was no correlation between serum folate and haematocrit or reticulocyte count. The erythrocyte folate, measured before and after removal of reticulocytes, remains a reliable indicator of deficiency of folic acid in sickle-cell patients despite variable degrees of reticulocytes. Folate therapy resulted in higher haematocrits in three of four sickle-cell patients with low serum and low erythrocyte folate values, but in only one of 12 patients with low serum folate and normal erythrocyte folate. These findings are consistent with other observations that folate requirement may be increased in chronic haemolytic disorders and suggest that erythrocyte folate values should be obtained in sickle-cell patients suspected of having folate deficiency.