Serum lipids and the cholesterol concentration in the high density lipoprotein (HDL) fraction were measured in epileptic patients taking either carbamazepine or phenytoin as single drug treatment. There was a significant increase in HDL-cholesterol levels in patients taking both phenytoin and carbamazepine. Serum total-cholesterol concentrations in patients taking both drugs did not differ significantly from those of controls. The ratio of HDL-cholesterol/total cholesterol was increased in both drugs and the increase reached significance in patients taking phenytoin. There was a significant increase in serum triglycerides in females taking carbamazepine.