We have performed a randomised, prospective study to compare the efficacy and tolerability of sodium valproate (VPA) and lamotrigine (LTG) monotherapy, and their effects on circulating androgenic hormones, in newly diagnosed epilepsy. A total of 225 patients (116 male; median age 35 years, range 13-80 years) were followed-up at 6-weekly intervals until they reached an end-point (12 months' seizure freedom; withdrawal due to intolerable side-effects; lack of efficacy despite adequate dosing). Twelve month seizure-free rates were identical (47%) in the VPA (n=111) and LTG (n=114) treatment arms. More patients taking VPA withdrew from the study due to adverse events (26 VPA versus 15 LTG; p=0.046). Eight patients, all taking VPA, dropped out during the first 6 months due to weight gain. There were no changes in mean serum concentrations of testosterone, sex-hormone binding globulin and androstenedione or in the free androgen index after 6 or 12 months' treatment with either drug in 112 patients who fulfilled the criteria for hormone analysis. No difference in efficacy was found between VPA and LTG in our patients with newly diagnosed epilepsy. LTG appeared to be better tolerated. Neither drug appeared to alter the circulating levels of androgenic hormones.