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.