This study assesses the long-term results of jejunoileal bypass (JIB) in 43 prospectively followed patients whose surgical bypass remained intact. Follow-up was 12.6 ± 0.25 years from JIB. Weight loss and improved lipid levels, glucose tolerance, cardiac function, and pulmonary function were maintained. Adverse effects such as hypokalemia, cholelithiasis, and B12 or folate deficiency decreased over time. The incidence of diarrhea remained constant (63% vs 64% at five years), while the occurrence of hypomagnesemia increased (67% vs 43% at five years, P < 0.05). Nephrolithiasis occurred in 33% of patients. Hepatic fibrosis developed in 38% of patients and was progressive. Overall, after more than 10 years, 35% of patients appeared to benefit from JIB as defined by alleviation of preoperative symptoms and the development of only mild complications (vs 47% at five years). On the other hand, irreversible complications appeared to outweigh any benefit derived from the JIB in 19% (vs no patients at five years; P < 0.01). In summary, patients with JIB remain at risk for complications, particularly hepatic fibrosis, even into the late postoperative period.