The surgical indications in 770 patients with Crohn's disease undergoing intestinal resection at The Mount Sinai Hospital from 1960-83 have been reviewed. Surgical indications were divided into two principal categories: 375 cases with perforating indications and 395 cases non-perforating. Among 292 patients who underwent second operations for recurrent Crohn's disease, the indications for second operation were closely dependent on the indication for primary resection. Second operations were undertaken for perforating indications much more often among cases where the initial indication had been perforating, than among those whose initial indications had been non-perforating (73% v 29%, p less than 0.00001). This trend to similarities in the indications which bring patients to surgery was maintained within each anatomical category of Crohn's disease and even between second and third operations (p less than 0.001). Operations for perforating indications were followed by reoperation approximately twice as fast as operations for non-perforating indications, whether going from first to second operation (perforating 4.7 v non-perforating 8.8 years, p less than 0.001), or from second to third (perforating 2.3 v non-perforating 5.2 years, p less than 0.005). Crohn's disease thus seems to occur in two different clinical patterns, independent of anatomic distribution. These are a relatively aggressive perforating type and a more indolent non-perforating type, which tend to retain their identities between repeated operations and to influence the speed with which reoperation occurs.