The patient was a 54-year-old woman with autosomal dominant polycystic kidney disease (ADPKD) who developed complicating systemic sarcoidosis. Hypercalcemia and an abrupt increase in serum creatinine levels were observed during the clinical course. Steroid therapy was initiated and produced a distinct improvement in renal function. A kidney biopsy was not feasible because ADPKD is a contraindication for renal needle biopsy. The clinical findings strongly suggested renal disorder secondary to tubulointerstitial nephritis caused by renal sarcoidosis with complicating hypercalcemia. In addition to controlling hypertension and improving the hypercalcemia and dehydration, steroid therapy also improved the renal function in this patient.