The effect of carbamazepine monotherapy on the red cell folate level of 133 patients with trigeminal neuralgia was evaluated. The patient group had a significantly lower mean value of red cell folate levels compared with 110 controls. No significant correlation was found between the red cell folate levels and the mean cell volume or haemoglobin values in either the carbamazepine or control group. In addition no significant correlation was found between the red cell folate levels and drug dosage. Administration of folic acid supplements raised the mean value of red cell folate significantly. Dietary folate intake was assessed in 43 trigeminal neuralgia patients and 33 matched control patients and there was no significant difference between the groups. Patients taking carbamazepine should be advised on a well-balanced diet rich in folate as opposed to being given a routine prescription of folic acid.