Histologic findings in liver biopsy specimens obtained from 88 patients before and one and two years after end-to-end jejunoileal bypass are compared. In addition to the expected fatty changes, mild changes of centrilobular, pericellular fibrosis were present in the initial biopsies in 8.6%; a year later they had become apparent in 46%. Portal-central bridging developed in 6.8%, and early micronodular cirrhosis in 3.4%--always in those with central pericellular fibrosis. Electron-microscopic study of pre-bypass liver biopsies from eight addtional patients showed collagen and electron-dense material resembling basement membranes within the spaces of Disse in seven, although only four had light-microscopic evidence of minimal central pericellular fibrosis. The existence of these light- and electron- microscopic changes before jejunoileal bypass suggests that there is a lesion in morbid obesity that may be exacerbated during the first year after operation.