Monoclonal IgG gammopathy of the lambda light-chain type was detected in a 51-year-old woman who had unexplained fever, muscle fatigue, and myalgia. One year later, myasthenia gravis was diagnosed. There was no evidence of myelomatosis or other malignancy. On the assumption that her M-component (gammopathic paraprotein) was related to myasthenia, she was treated with melphalan and cyclophosphamide, but her clinical condition was not improved. Despite therapeutic trials of other agents and a time course of 6 years, the quantity of the M-component remained unchanged. Serum AChR antibody activity was not located in the paraprotein peak. The findings do not support a relationship between the M-component and myasthenia gravis.