A field mapping technique was used to detect changes in the local magnetic field in 14 tumors, five hematomas, one lacunar infarct, and two cases of multiple sclerosis. These changes were attributed to variations in the tissue susceptibility. A partial or complete rim of changed susceptibility was detected at the margin of five of the tumors and localized changes were seen within two of the tumors. All five hematomas displayed susceptibility effects. These were seen at the rim of the hematoma in all cases but central effects were also seen in four cases. One area of probable lacunar infarction also displayed susceptibility effects. The effects described may be due to the paramagnetic contribution of species such as deoxyhemoglobin, methemoglobin, free ferric iron, hemosiderin, and other breakdown products of blood. Local field inhomogeneities due to changes in tissue susceptibility have implications for magnetic resonance (MR) image interpretation and the performance of MR spectroscopy.