Nine patients with polycystic kidney disease were examined by MRI, CT, and sonography. MRI distinguished among simple cysts, cysts complicated by prior hemorrhage, and coexistent renal cell carcinoma. On T1-weighted spin-echo images, simple renal cysts appeared as round, homogeneous, low-signal regions with smooth outer margins and a distinct interface with remaining normal renal parenchyma or adjacent cysts. Hemorrhagic cysts were seen as homogeneous medium- to high-signal intensity regions, and about half of them correlated with hyperdense cysts by CT. In five cases, fluid-iron levels were evident by dependent high-intensity layering within the cysts. Renal cell carcinomas occasionally show high intensity because of hemorrhage, but intratumoral fluid-iron levels have not yet been described. These results suggest that MRI is useful in differentiating between simple cysts, hemorrhagic cysts, and neoplasms when CT and sonography are indeterminate.