Sera from 2 patients with ovarian carcinoma and paraneoplastic cerebellar degeneration confirmed postmortem were reacted with frozen sections of human cerebellum and stained using indirect immunofluorescence methods. Both sera produced bright cytoplasmic staining of Purkinje cells and of neurons within deep cerebellar nuclei. Titration of these sera to end point revealed staining at final dilutions of 1:640 and 1:2,560, respectively. Neither of these sera reacted with sections of human cerebrum, basal ganglia, spinal cord, peripheral nerve, lung, liver, kidney, or ovary. Staining of Purkinje cells was not obtained with sera from 34 normal, healthy controls, 5 patients with oat cell carcinoma of the lung, 6 patients with inflammatory central nervous system disorders, or 12 of 14 neurologically normal patients with ovarian carcinoma. Sera from 2 neurologically normal patients with ovarian carcinoma, however, produced staining of Purkinje cells and deep nuclei similar to that obtained with sera from patients with paraneoplastic cerebellar degeneration. The present study documents the presence of antibodies to Purkinje cells in patients with ovarian carcinoma and cerebellar degeneration and demonstrates that development of these antibodies may antedate the onset of clinically evident cerebellar degeneration.