We describe 2 patients with myasthenia gravis and non-Hodgkin's lymphoma outside the thymus gland, in whom the two diseases progressed at different rates. Diagnosis of myasthenia gravis was based on clinical signs and compatible neurophysiologic studies, specifically by high acetylcholine antireceptor titers in the first patient and a positive Tensilon test in the second. In the first patient the clinical and serological signs of the two diseases progressed similarly. The association of these two diseases may have been the result of an underlying immunological disorder favoring their appearance, or of an immune response, caused by the lymphoma involving postsynaptic nicotinic acetylcholine receptors.