Acquired renal cystic disease (ARCD) has a prevalence of up to 90% in patients with endstage renal failure and an uncommonly high potential of developing into renal cell carcinoma. After renal transplantation, regression of an established ARCD is possible, suggesting a protective effect of transplantation against tumors in the native kidneys. Two case reports describing hypernephromas in kidneys with ARCD 3 years after successful renal transplantation are presented. One patient died 6 weeks after nephrectomy due to metastatic disease, although there were no metastases at the time of operation. The other patient lives with no evidence of disease since 10 months. This report confirms the need of annual sonography of the native kidneys also in renal transplant patients with consecutive computed tomography scanning of suspicious lesions.