This paper reports a double-blind placebo-controlled trial of oral tobramycin in acute ulcerative colitis. Eighty-four patients with an acute relapse of ulcerative colitis were randomized to receive oral tobramycin or placebo for 1 week as an adjunct to steroid therapy. At endpoint, 31 of 42 (74%) in the tobramycin group achieved complete symptomatic remission compared with 18 of 42 (43%) in the placebo group (P = 0.008). The tobramycin group achieved better histological scores (P less than 0.05) at endpoint. These findings show that treatment with oral tobramycin improves the short-term outcome of patients with ulcerative colitis in relapse.