A histomorphometric analysis of the superficial peroneal nerve was made in 15 cases of leprosy. Thirteen patients presented clinical signs of leprous neuropathy, while the other two showed only cutaneous signs of leprosy. The presence of M. leprae in all the nerves sampled, and the appearance of the histologic lesions, made it possible to confirm the diagnosis of leprosy and to specify the type of leprosy in each case, even in the absence of clinical signs of leprous neuropathy. Correlation of the histomorphometric results with the duration of development of the disease and with the time elapsing before treatment showed, in the beginning stage, a considerable reduction in myelinated and unmyelinated never fibres and a proliferation of Schwann cells, as well as segmental demyelination and axonal degeneration of the teased fibres. When treated early, the evolution of the lepromatous type (LL) appears favourable, with apparent regeneration of the nerve fibres. When treatment is not instituted early, gradual loss of nerve fibres, axonal degeneration of all the teased fibres, proliferation of the Schwann cell processes devoid of axons, and increase in endoneural connective tissue lead to a severe degeneration of the nerve. This unfavorable development appears to progress faster in the absence of treatment or when treatment is irregular. In the borderline lepromatous (BL) and borderline tuberculoid (BT) types, nerve degeneration appears to be more rapid than in the type LL.