A brief illustrated account is presented of the light microscopic pathology, histochemistry of lysosomal enzymes, and fine structural changes in the nerves of patients with untreated or treated lepromatous leprosy. Predominant bacillation of the Schwann cells of unmyelinated fibres, degeneration of their axons, prominence of phagolysosomes, and disappearance of these cells with endoneurial collagenosis were observed on electronmicroscopic examination of the index branch of the radial cutaneous nerve. Although there were changes in the blood vessels and proliferation of perineurium, bacillation of endothelial or perineurial cells was much less conspicuous. Intact and degenerating forms of M. leprae were found in both treated and untreated patients, fragmenting or crumpled forms being more frequent in the treated. Both groups of patients also showed increased lysosomal enzyme activity, evidenced by single or paired paranodal spots of acid phosphatase and beta-glucuronidase in Schwann cells in histochemical preparations of the nerve. There was lesser activity, and activity in fewer cells, in the case of beta-glucuronidase than of acid phosphatase. Diffuse beta-glucuronidase activity was found in the wall of empty-looking oval chambers in the Schwann cells, and acid-fast bacilli were seen in these chambers. In teased fibre preparations, both axonal degeneration and segmental demyelination were found. In semi-thin araldite sections, the myelinated fibre density was either preserved or reduced; large diameter fibres were more frequently depleted, with tall peaks of smaller fibres seen on plotting diameter spectra.