Despite circumstantial evidence that opsoclonus-myoclonus (OM) is often immune mediated, no specific autoantigen has been identified. Using sera of 21 patients with several types of OM (idiopathic, associated to small cell lung cancer, and associated to neuroblastoma), we probed a brainstem cDNA library to isolate target neuronal antigens. Thirty-seven clones coding for 25 proteins were isolated, with two groups of autoantigens emerging: (1) proteins of the postsynaptic density, among them the adenomatous polyposis coli, and 2) proteins with expression or function restricted to neurons, including RNA or DNA-binding proteins and zinc-finger proteins. Usually, each patient's serum recognized a different autoantigen, except for adenomatous polyposis coli that was recognized by sera of two patients with idiopathic OM and two control patients with nystagmus, diplopia, and paraneoplastic brainstem dysfunction. Overall, in the indicated types of OM, (1) we found frequent and heterogeneous immunity to neuronal autoantigens without a single specific antibody marker of OM, (2) the occasional detection of antibodies to known onconeuronal antigens (ie, Hu proteins) probably is related to cancer-induced immunity rather than to OM, and (3) the postsynaptic density is a frequent source of novel autoantigens, with several proteins of this complex targeted by antibodies of OM patients.