A randomized, comparative trial was carried out in 100 patients with trichomonal vaginitis to compare the efficacy and toleration of a single 2 g dose of tinidazole and metronidazole. Tinidazole produced parasitological cure in 94% of patients (47/50) and a satisfactory clinical response in 96% of patients (48/50); for metronidazole the figures were 64% (32/50) and 72% (36/50) respectively. Side-effects were reported by 52% of patients (25/50) on tinidazole and 82% of patients (41/50) on metronidazole. Severity of side-effects and their frequency per patient were significantly less (p less than 0-045 to 0-01) with tinidazole than with metronidazole. Statistical evaluation of the results showed that tinidazole was significantly better than metronidzole with respect parasitological cure (p less 0-01) and clinical improvement (p less than 0-01) and also with regard to the incidence and intensity of side-effects (p less than 0-01).