Carbamazepine is chemically related to imipramine. It can reduce prostaglandin E2-like activity in inflammation. It caused overflow urinary incontinence, increased bladder capacity, sensitized renal tubules to antidiuretic hormone and leading to antidiuresis. This encouraged to use carbamazepine to treat primary enuresis. Twenty-six patients of either sex with a history of enuresis from birth were included in study. Their age ranged between 7 and 15 years (mean 9.3 years). They were assessed by history, physical examination, blood glucose, renal function tests, intravenous urogram and videocystourethrography. 30 days drug-free observation was performed to establish baseline voiding pattern. This was followed by two, 30 day treatment periods of either placebo or carbamazepine (200 mg) tablets, in a randomized, double-blind cross-over design. There was one week washout period between medications. The patients or their parents received calendar sheet to record wet and dry nights and offered subjective opinions concerning changes in sleep patterns, occurrence of nocturia and appearance of side-effects. A tablet was given to each patient before retiring. Those patients who showed no response to carbamazepine and placebo would be treated with 100 mg of indomethacin suppositories. The results show that of 26 patients 20 had 7 to 30 of 30 dry nights with carbamazepine, while 6 had 0 to 5 of 30 dry nights. The latter 6 patients reacted in the same manner with placebo, 4 of them showed better response with use of indomethacin. Six patients had 10 to 15 of 30 dry nights during placebo therapy and 20 had 0 to 6 of 30 dry nights. The mean number of dry nights was 3.92 +/- 5.22 with placebo and was 18.8 +/- 8.82 with carbamazepine. The difference in response to placebo and carbamazepine was statistically significant (p < 0.001). All the patients who responded sufficiently to indomethacin slept until the morning. No side effect was noticed with either treatment and repeated serum electrolytes and other laboratory tests were normal after treatment. It might be concluded that carbamazepine is useful for treatment of primary nocturnal enuresis.