Objective To compare the effectiveness and adverse effects of nifedipine versus indomethacin in the treatment of preterm labor. Methods In a randomized clinical trial, 79 women with labor pain at 26โ33 weeks of gestation were treated with either oral nifedipine (n = 40) or rectal indomethacin (n = 39). Results Twenty-three (59%) women in the indomethacin group, and 10 (25%) in the nifedipine group did not respond to treatment (P = 0.002). None of the 16 and 30 women remaining in the indomethacin and nifedipine groups, respectively, delivered during the subsequent 48 hours. Of these remaining women, 1 (6.25%) in the indomethacin group and 4 (13.3%) in the nifedipine group delivered between 48 hours and 7 days (P = 0.162). For the women who responded to treatment, the mean gestational age at time of delivery was 238.5 ± 19.4 days and 246.4 ± 15.4 days in the nifedipine and indomethacin groups, respectively (P = 0.182). Seventeen (42.5%) women in the nifedipine group, and 11 (28.2%) in the indomethacin group showed adverse effects (P = 0.184). Conclusion Indomethacin was less effective than nifedipine for the fast treatment of preterm labor. For women who responded to treatment within 2 hours, however, the delaying of delivery by indomethacin was similar to that by nifedipine.