In order to assess the ability of magnetic resonance (MR) imaging to distinguish simple renal cysts from renal tumors, images from 53 patients with 74 lesions were reviewed. MR image characteristics, including signal strength and homogeneity for each lesion, were evaluated and compared with the final diagnoses. There were 53 cysts and 21 tumors. For 56 lesions (76%), MR allowed accurate diagnosis; on T1-weighted images, homogeneous low-intensity lesions were always cysts and inhomogeneous medium- or high-intensity lesions were always tumors. The remaining 18 lesions (24%) could not be distinguished, and T2-weighted images failed to improve differential diagnostic ability. In our experience, MR imaging allows differentiation of renal cysts and tumors less frequently than does computed tomography (CT) or ultrasound (US). MR criteria for simple renal cysts and tumors are offered, with the understanding that modification may be necessary as experience with these lesions increases.