During the course of a larger study of patients with treated lepromatous leprosy attending a hospital in South India, sixteen patients with untreated disease presented with symptoms suggesting both intranasal and sinus involvement. Radiological abnormalities of the sinus were recorded in all sixteen patients and the results have been published separately. This paper describes the clinical and bacteriological findings, together with the results of histopathological examination of biopsies of sinus mucosa in two patients, in both of whom there was a histiocytic infiltrate, with acid-fast bacilli. In one of these biopsies, bacilli in the lamina propria were numerous; globi were common, and 20% of free-standing organisms were solid-staining. The significance of these findings is discussed with emphasis on the value of a full ear, nose and throat examination in all cases of leprosy, especially those with the lepromatous form. In this type of leprosy the upper respiratory tract tissues are clearly of importance for the lodgement and of multiplication of bacilli; it is possible that the paranasal air sinuses, with their large surface area, may contribute to the numbers of bacilli which are disseminated from the nose into the environment.