To assess the diagnostic performance of multislice computed tomography (MS-CT) in the classification of atypical or complex cystic renal masses using the Bosniak system in comparison to contrast-enhanced ultrasound (CEUS) and, in unclear cases, to the surgery findings.Thirty-two consecutive patients (14 women, 18 men; age range 39-72 years) with 37 atypical or complex cystic renal masses at MS-CT underwent conventional ultrasound (US) and CEUS. CEUS employed a low-MI technique using 1.6-2.4 ml SonoVue (Bracco, Italy) i.v. and a 2-4 MHz multifrequency transducer (Siemens, Sequoia, Acuson). Fourteen masses were resected, the remaining 23 lesions were followed up for periods ranging from 3 months to 2 years. Images and digital cine clips of all lesions were evaluated by blinded readers. On the basis of MS-CT appearance the lesions were assigned to the Bosniak classification. Similar criteria modified for US imaging were used to score atypical cysts at CEUS. In the Bosniak classification at MS-CT the lesions were scored as category II (n=15), IIF (n=7), III (n=8) and IV (n=7). At CEUS, masses were classified as Bosniak classification II (n=8), IIF (n=12), III (n=8) or IV (n=9). All type IV and 6/8 type III and 1/8 type IIF lesions were removed surgically. All category IV and 3/8 category III lesions of the surgical group were malignant, the one type IIF lesion was benign. All class II and IIF cysts except one were stable after a follow-up period ranging from 3 months to 2 years. In 7/37 lesions (19%) the MS-CT and CEUS scores were different, while in 30/37 (81%) they were equivalent. CEUS depicted more thin septa than MS-CT, or upgraded wall thickness, resulting in a Bosniak score upgrade from category II to IIF in 5 lesions. Two cystic renal masses could not be clearly assigned by MS-CT but were considered malignant due to the additional information from CEUS, which was confirmed by surgical removal (small cystic renal cancer).CEUS with SonoVue allows an early evaluation of atypical or complex cystic renal masses. It is an additional examination to MS-CT. Due to the dynamic examination, additional information about perfusion of the cystic septa or cystic renal cancer can be gained.