An absorptive, hormonal, bacteriological and morphological study was performed in 9 proctocolectomized patients with continent ileal reservoirs established for 2-57 months. Vitamin B 12 malassimilation (as assessed by the Schilling test and serum vitamin B 12 concentration), bile salt deconjugation (modified 14C-glycocholate breath test), increased stool fat excretion and high intestinal sodium loss with secondary hyperreninism/hyperaldosteronism were encountered in a large percentage of patients; proximal small intestinal absorption (serum parameters, xylose test) appeared intact. In reservoir stool samples total and differential bacterial cell counts yielded values close to a normal rectum flora. Morphologically (endoscopy, quantitative histology) the reservoir mucosa showed moderate to severe inflammation; structural changes were essentially absent and inflammation in the adjoining ileum was minimal. These results indicate that reservoir bacteria frequently produce disturbances in small intestinal function and that these may be found without secondary structural transformation of the mucosa.