A 38-year-old woman showed symptoms of myasthenia gravis (MG) three months after receiving thymectomy for malignant thymoma. She was treated with anti-acetylcholine esterase drugs and azathioprine over 10 years with two exacerbations, which were controlled by plasmapheresis and large amounts of steroid. Nephrotic syndrome developed suddenly at the age of 48, was progressive even after azathioprine withdrawal, and resistant to several immunosuppressive therapies such as steroids and cyclosporine A, and plasmapheresis. She died of systemic infection one-and-a-half years after the onset of nephrotic syndrome. Immunological studies revealed several abnormalities of cellular immunity. The expansion of gamma-delta T cells and CD57+ lymphocytes in peripheral blood was characteristic findings. These cells are thought to originate from the extrathymic process. Nephrotic syndrome has been thought to be sometimes complicated with thymoma. Although some pathogenetic possibilities about combination of nephrotic syndrome and thymoma were supposed, none has yet been clarified. As we noticed the remarkable increase in the number of CD57+ cells, we examined its proportion in the peripheral blood of patients with MG and/or thymoma, as well as in individuals without sickness. The study revealed the expansion of CD57+ cells in MG thymoma patients (32.3 +/- 15.9%), compared with healthy controls (15.2 +/- 5.4%), MG non-thymoma patients (20.3 +/- 11.5%), and thymoma non-MG patients (15.2 +/- 12.0%) statistically (Mann-Whitney's U test). Therefore, we supposed that the peripheral CD57+ cell expansion was associated with parathymic immunological abnormalities, such as MG, thymoma, and nephrotic syndrome.