We followed 21 patients with sensory neuronopathy without evidence of cancer for up to 23 years. All were seronegative for type 1 antineuronal nuclear antibodies (ANNA-1, also called “anti-Hu”). We additionally studied 67 seropositive patients with sensory neuronopathy or a related neurologic syndrome. Ninety-one percent of the seropositive patients had a small-cell lung carcinoma. One, with a normal chest x-ray, had been followed for 7 years for sensory neuronopathy of indeterminate cause before serologic testing for ANNA-1 led to the discovery of the tumor by CT. We conclude that ANNA-1 seropositivity in a patient with sensory neuronopathy is strong evidence for an underlying small-cell lung cancer.