Neuron-restricted autoantibodies are important markers of neurological autoimmunity related to cancer. We identified a new paraneoplastic IgG, PCA-2 (Purkinje cell cytoplasmic antibody type 2), in 10 patients. Nine had mixed subacute neurological presentations (5 brainstem or limbic encephalitis, 3 cerebellar ataxia, 2 Lambert-Eaton myasthenic syndrome, 1 autonomic neuropathy, and 1 motor neuropathy). All 9 were smokers, and 8 had definite or probable lung cancer (7 with biopsy-confirmed small cell lung carcinoma [SCLC]; 1 imaged only). One patient had no follow-up information. A 10th patient was among 58 with uncomplicated SCLC. PCA-2 binds to a cytoplasmic antigen in neurons and SCLC cells. Its immunostaining pattern in mouse tissues is distinct from that of the paraneoplastic autoantibodies PCA-1 (anti-Yo, marker of immune response initiated by ovarian or breast carcinoma) and PCA-Tr (anti-Tr, immune response marker of Hodgkin's lymphoma). PCA-2 binds to cerebellar Purkinje somata and dendrites, neurons in internal granular layer and dentate nucleus, and neuronal elements in gut and kidney. Western blots of reduced/denatured cerebellar and SCLC proteins reveal a common antigenic band, of approximately 280 kd. PCA-2 is the seventh IgG neuronal autoantibody marker of paraneoplastic autoimmunity identifiable unambiguously by standardized immunofluorescence criteria.