The percentage of lymphocytic subsets in the blood of cases with neuralgic amyotrophy (NA), and the proliferative response of blood lymphocytes cultured with different nerve extracts, obtained from normal subjects at postmortem, were examined in 6 patients with NA and in 18 age-matched controls with shoulder pain not related to NA. Most (5/6) NA patients had decreased CD3 values and increased CD4/CD8 ratios due to a decreased of the CD8 subset. Lymphocytes of NA patients increased their blastogenic activity in cultures with nerve extracts from different brachial plexus nerves and its branches, but not in cultures with extracts of sacral plexus nerves. Cultures did not respond to nerve extracts in any of the control cases, although mitogenic activity was similarly elicited in cultured lymphocytes stimulated with phytohemagglutinin in both control cases and NA patients. These results suggest that NA is probably an immune mediated disease.