Recurrence rate is high after operation for Crohn's disease. A multicenter trial was performed to study the effect of radical or nonradical operation and of sulfasalazine prophylaxis versus placebo on postoperative recurrence rate in 232 patients with Crohn's disease. Sixteen medical and surgical centers participated in the study, 7 operating radically and 9 nonradically. The follow-up period lasted 3 years, the allocation to drug treatment was randomized and double blind. Recurrence was significantly less frequent and occurred later in patients who were operated nonradically. Patients on sulfasalazine prophylaxis had a better prognosis than on placebo. This effect was statistically significant in the first 2 years of treatment. Both strategies were additive: nonradical operation and sulfasalazine had the best prognosis, radical operation and placebo was worst. It is concluded that postoperative recurrence is best prevented by resecting nonradically and prescribing 3 g of sulfasalazine daily at least over 2 years.