In a double-blind, randomized trial, we tested the effectiveness of metronidazole (0.6 g/day) against sulfasalazine (2 g/day) in the maintenance of remission in patients with ulcerative colitis. The patients were in remission for 1-11 months at entry to trial, which lasted for 12 months. Forty patients entered the trial and 33 completed it. Metronidazole was found to be slightly more effective than sulfasalazine, a difference statistically significant only at 12 months. Six patients also completed a crossover trial. Remission was maintained for 12 months in 3 patients by metronidazole and in none of the 6 by sulfasalazine. No significant side effects were noted, and in particular, no paresthesias were reported. This trial, as well as our previous one, suggests that metronidazole may be useful in the maintenance of remission in patients with ulcerative colitis, but that it is ineffective in the therapy of the acute attack.