A patient presented with anti-acetylcholine receptor antibody-positive myasthenia gravis. After removal of a thymoma and use of cytotoxic therapy, there was worsening of myasthenia, onset of muscle stiffness and hyperexcitability, and electrophysiologic signs of peripheral neuropathy. Elevated serum titers of antibodies to neuronal voltage-gated K+ channels were present, consistent with neuromyotonia (Isaacs' syndrome). A beneficial response to treatment paralleled changes in antibody titers.