Irregularly structured brain tumors, such as glioblastomas, challenge attempts to visualize and quantify their three dimensional structure. Magnetic resonance imaging (MRI) represents one tool for attempting to noninvasively track tumor size. MR images demonstrate widely varying perceived tumor margins. In addition, adjunct therapies, such as the administration of steroids, greatly affect the volumes perceived in images formed by certain pulse sequences. In this study tumors were grown in 15 dogs and the tumor size tracked for a period of time. The dogs were placed on dexamethasone for a week and another series of scans was obtained. No other therapies were provided. The data for visualized tumor size are provided for T1, T2, and proton density. Weighted images are provided and the relationships between the scans are discussed.