In 47 patients who underwent 53 liver transplantations and immunosuppression with cyclosporine (cyclosporin A), methylprednisolone sodium succinate, and antithymocyte globulin, 146 histopathological studies were performed (138 biopsies, six hepatectomies, and two autopsies). The following microscopical diagnoses were made: 43 acute rejections (29.4%), six chronic rejections (4.1%), 18 liver blood perfusion changes (12.3%), 15 biliary changes (10.2%), 10 cases of functional cholestasis (6.8%), two drug reactions (1.3%), two hepatitis B virus recurrences (1.3%), 11 opportunistic viral infections (7.5%), 18 minimal changes (12.3%), two nonclassifiable changes (1.3%), and 19 plurietiological changes (13%). A histopathological diagnosis of acute rejection was made in 31 transplants (58.4%). In 22 (71%) of them, acute rejection was diagnosed with the protocol biopsy specimen that was obtained during the second posttransplant week. Leukocyte counts and serum bilirubin and enzyme levels were obtained on the same day that the hepatic biopsy specimens were taken. There was no significant statistical difference between the mean serum data that accompanied each histopathological diagnosis, allowing identification of a characteristic biochemical profile for the causes of graft dysfunction. We report a detailed description of the microscopical findings of each diagnosis and the following conclusions: (1) Acute rejection is the most frequent cause of hepatic dysfunction and has an early appearance during the posttransplant period. (2) Histopathological findings can identify the causes of the dysfunction. (3) There is no specific biochemical pattern to differentiate these causes. This may be due to the frequent combination of etiological factors in every dysfunction episode.