Two cases of end-to-end and end-to-side jejuno-ileal bypass for the treatment of obesity are reported in which both patients underwent reoperation 17 and 23 months later respectively. Macroscopic examination and measurements of length of the various segments of the small intestine have been performed twice in each patient: at the time of the bypass and at the second surgical procedure. In one patient a histological study of both the excluded and functioning small intestine was carried out at the time of each surgical procedure. The functioning intestine had increased in size while the excluded segment had become narrowed. The height of the villi had increased slightly in the functioning ileum and more so in the functioning jejunum compared with the preoperative measurements. In contrast, the height of the villi was moderately reduced in both the bypassed jejunal and ileal segments. These findings confirm the results of experimental studies in animals.