Twenty-five patients with ulcerative colitis considered "failures of therapy with adrenal steroids and salicylazosulfapyridine" but without absolute indication for surgical intervention were treated with 6-mercaptopurine. The dose ranged from 50 to 200 mg daily and the period of treatment from 10 to 52 months. At followup 15 were completely well and 8 improved; 2 required definitive surgery. Twelve had not required steroid therapy for 1 to 4 years and 8 received some form of maintenance therapy intermittently or a small sustained dose. On barium enema examination the colon was improved in 13 of 18. Transient vomiting complicated therapy in 9, and leukopenia occurred 38 times in 24 patients. There were no serious complications. The results still suggest that responses to 6-mercaptopurine are not as dramatic as those observed with steroids, but the former drug has a sustaining or supplementary role. It serves to decrease the frequency and severity of recurrences and eliminates or remain the need for steroids. Objections to 6-mercaptopurine therapy remain theoretic, and the drug seems sufficiently safe to use for periods as long as 4 years. Sufficient controlled studies are still lacking and should be encouraged.