Paraneoplastic syndromes may affect the central and peripheral nervous system of adults and children with cancer. Neurological symptoms may resolve with treatment of the underlying neoplasm. We report the case of a child with Wilms tumor who presented with generalized weakness, fatigue, ptosis, hypokinesis, dysarthria, urinary retention, facial diplegia, ophthalmoplegia, and autonomic dysfunction. Routine electrodiagnostic testing, including repetitive nerve stimulation, was normal. Clinical features and stimulation single-fiber electromyogram were consistent with a neuromuscular junction transmission disorder, likely Lambert-Eaton myasthenic syndrome. The child's neurological status returned to normal with successful treatment of the tumor.