We describe the critical role of MR imaging in a case of postpartum ovarian vein thrombosis (OVT) with concomitant pyelocaliceal ectasia. MR imaging confirmed the diagnosis suspected on the basis of ultrasonography and computed tomography by demonstration of a subacute clot with high signal intensity within the right ovarian vein and its complete resolution after anticoagulant therapy. MR imaging is a useful noninvasive, accurate tool for the diagnosis and follow-up of this potentially life-threatening condition, providing information helpful for choosing a prompt medical treatment rather than a surgical therapy. To our knowledge, no previous case of OVT causing pyelocaliceal ectasia documented by MR imaging has been reported. However, even though our case is suggestive, a cause-effect relationship between OVT and hydronephrosis could not be demonstrated with certainty.