A 23-year-old man was admitted to our hospital for fatigue, anorexia, proteinuria, and peripheral edema. Proteinuria was first pointed out at the age of 15, but no further studies were performed. Six years prior to admission, the patient noted becoming easily fatigued. Laboratory tests on admission showed marked peripheral eosinophilia (29.2%, count: 1,071/microL) and proteinuria. Endocrinological studies revealed isolated adrenocorticotropic hormone deficiency with primary hypothyroidism, but a lack of autoimmune thyroiditis. Renal biopsy showed focal segmental glomerulosclerosis. Hydrocortisone therapy (30 mg/day) for isolated adrenocorticotropic hormone deficiency resulted in marked improvement of adrenal and thyroid functions, and amelioration of proteinuria (from 2.8 to 1.0 g/day) over a two-month period. Renal function remains normal at five years after the start of hydrocortisone treatment. The findings suggest that both hydrocortisone therapy and normalized thyroid hormone status played a pivotal role in the improvement of proteinuria associated with focal segmental glomerulosclerosis.