Myasthenia gravis and lymphoma rarely coexist, but the occurrence of myasthenia shortly after the treatment of a patient with poorly differentiated nodular lymphoma suggested that an immunological disorder may have contributed to the development of both diseases; the fundamental defects in this association may be impaired immunological surveillance and impaired regulation of immune responses to autoantigens. The finding of T-cell immunodeificiency, including profound T-cell lymphopenia, impaired delayed hypersensitivity responses, and failure to a thymus-dependent antibody response to Salmonella adelaide flagellin, is consistent with this hypothesis.