Histologic findings in liver biopsy specimens obtained from 88 patients before and 1 and 2 yr 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 8 additional patients showed collagen and electron dense material resembling basement membranes within the spaces of Disse in 7, although only 4 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.