The sensory loss which occurs in leprosy is essentially cutaneous, resulting from centripetally ascending infection, the host cellular response and fibrosis, from dermal to certain mixed nerves. The hallmark is pain/temperature and touch/pressure loss. Muscle denervation is a byproduct of mixed nerve involvement. Leprous sensory and motor neuropathy presents a stereotyped picture, with preservation of position sense, noninvolvement of the large girdle muscles, and retained deep tendon reflexes. We report clinical and investigative details of 7 patients (3 males, 4 females) with mild-to-moderate polyneuritic leprosy who manifested severe proprioceptive loss in the upper per limbs; the lower limbs were similarly affected in 4 of them. Tendon reflexes were absent in the ataxic limbs. No other cause was found for the ataxia. Electrophysiological studies confirmed damage to large cutaneous and muscle afferents, and a normal EMG pattern in hip and shoulder muscles. Of great interest was the histology of a lumbar sensory ganglion biopsied in a severely disabled patient. There was extensive neuron loss and degeneration and reactive proliferation of capsular cells ("nodules of Nageotte"), an inflammatory focus of lymphocytes, and no bacilli. This suggests to us that the proprioceptive loss in these patients could well be the result of an unusual "leprous ganglionitis." Further clarification of the mechanism of ganglion degeneration and the frequency of inflammation could come from immunohistology of tissues from African green monkeys with experimental polyneuritic leprosy.