A six-week placebo-controlled trial of the efficacy and safety of 6 g per day of 4-aminosalicylic acid (4-ASA) was conducted in 30 subjects with mild to moderately severe ulcerative colitis. Subjects were stratified into groups having distal (< 60 cm) or more extensive (> 60 cm) disease. Diarrhea, bleeding, sigmoidoscopic and biopsy appearance, and physician global assessment were scored to judge efficacy. Safety was evaluated by monitoring untoward symptoms and laboratory values. Median percent improvement was significantly greater (P < 0.05) in the 4-ASA > 60-cm group (42.7%) than in the placebo > 60-cm group (21.2%), but 4-ASA was not better than placebo for the < 60-cm group or the total study group. Severe dyspepsia (one subject), abnormal AST (transient in five, persistent in one) and elevated lipase without pancreatitis (six subjects) were noted. Thus 6 g 4-ASA for six weeks was more effective than placebo in mild to moderate ulcerative colitis extending more than 60 cm above the anus, but not in distal disease, and the drug was generally well tolerated.