The authors report a case of a renal cell carcinoma occurring in an adult suffering from polycystic kidney disease and diagnosed by radiologic examination. The patient presented with palpable flank mass, hematuria and weight loss. Ultrasonography, CT, MRI and angiography were performed preoperetively. Although severe renal architectural distortion and destruction caused by polycystic disease had masked evidence of malignancy, this was diagnosed after examination of the solid component of the mass lesion, and its hypervascular character, as seen on the three types of radiologic image. Malignancy was confirmed through nephrectomy and pathological examination.