From a series of 343 nerve biopsies taken by one clinician over a period of 12 years in Nepal, this paper describes the histopathological findings in 153 biopsies from 119 patients suffering from tuberculoid, borderline (dimorphous) or lepromatous leprosy, who were untreated at the time of first presentation and diagnosis. They were taken during the course of other studies, mainly concerned with the mode of transmission of leprosy, and which included biopsies of skin, dartos muscle, nasal mucous membrane and nipple, results of which have already been published. Examination of serial sections by light microscopy revealed a density of cellular infiltration in non-lepromatous cases, or of bacilli in macrophages and Schwann cells in lepromatous cases, which was marked in degree and usually widespread from one end of the biopsy to the other. Intraneural caseation was recorded in four patients with tuberculoid or borderline-tuberculoid leprosy, and many others in this part of the spectrum showed extensive disruption of perineurial and endoneurial structure. In lepromatous patients, the numbers of bacilli in the endoneurial area not infrequently exceeded one thousand per oil immersion field. Although well known to histopathologists familiar with this disease, it is considered that the significance of these findings, in patients presenting for the first time, is not well appreciated by those working in general medicine, neurology, epidemiology, or even in leprosy control.