A 26-year-old woman who presented facial and lower leg edema associated with massive proteinuria was admitted to our hospital in February 1992. Nine months before this admission, she exhibited myasthenia gravis and malignant thymoma, and underwent total thymectomy. On admission, there was no symptom of myasthenia gravis. She was diagnosed as having nephrotic syndrome and the first renal biopsy was performed. The histological findings showed membranous nephropathy. Immunofluorescent microscopy revealed that IgG and C3 were stained in a granular pattern in the periphery, and subepithelial deposits were observed in the basement membrane of the glomerulus by electron microscopy. With the administration of prednisolone, proteinuria disappeared and the nephrotic syndrome remitted. She was admitted again in January 1993 due to proteinuria and lower leg edema following cystitis. The findings of the second renal biopsy were unremarkable. She was administered cyclosporin A to improve the nephrotic syndrome and to reduce the side effects of prednisolone. The proteinuria disappeared again and this effect was dependent on the dose of cyclosporin A. Since the first administration, no symptoms of myasthenia gravis or malignant thymoma have been observed. The relationships among myasthenia gravis, malignant thymoma and nephrotic syndrome were examined. Although the first renal biopsy findings showed membranous nephropathy, from the therapeutic responses of both prednisolone and cyclosporin A, the main course of proteinuria in this case may have been due to minimal change nephrotic syndrome. We consider this case of nephrotic syndrome to be important considering its etiology and the relationship between the histological findings and its clinical course.