Summary The incidence of macrocytosis and low serum folate levels has been estimated in 115 mental hospital patients on anticonvulsant drug therapy and in 48 mental hospital control patients. Low serum folate levels were found in 36·1% of the patients treated with anticonvulsants. Of the patients on anticonvulsants, 14·6% had red cell macrocytosis, and the incidence of macrocytosis was significantly higher in female than in male patients. The mental hospital patients not on anticonvulsants who were used as controls had mean serum folate values significantly below that of the normal, non‐hospitalized controls. Attention is drawn to the influence of the diet on normal folate ranges, which must be taken into account during their assessment. The fact that a combination of factors is probably necessary to produce a frank megaloblastic change in anticonvulsant‐drug‐treated patients could account for the relative infrequency of megaloblastosis among these patients. It is believed that, in case of anticonvulsant drug therapy, a borderline folate diet often interacts with the mild drug folate inhibition. Folic acid absorption tests, carried out on 19 patients on anticonvulsant therapy with subnormal folate levels, showed slightly decreased absorption values at two hours after the oral test dose but no difference at three hours. The significance of these findings is discussed, and attention is drawn to possible mechanisms of mild malabsorption of folates in patients on anticonvulsant drugs.