We report the case of a 47-year-old patient with autosomal dominant polycystic kidney disease, who became symptomatic owing to a painful metastasis of a renal cell carcinoma. Abdominal sonography, multiphase CT and MRI were performed in order to localise and resect the primary tumour, but its identification was severely hampered owing to multiple renal cysts. In addition to standard MRI protocol, a spin-echo, single-shot echo planar diffusion-weighted imaging sequence was performed. The mean apparent diffusion coefficient (ADC) of renal cysts was 2.66±0.12 × 10(-3) mm(2) s(-1), 1.76±0.19 × 10(-3) mm(2) s(-1) of renal parenchyma in and 1.26±0.18 × 10(-3) mm(2) s(-1) of a suspicious soft-tissue mass at the midsection of the right kidney, indicating an area of higher cellularity. This value was significant lower than the ADC obtained for renal parenchyma (p<0.0001). Right-sided nephrectomy was performed and the pathological diagnosis of the suspicious lesion was clear cell renal carcinoma.