A method by which the tissue parameters - T1, T2, and proton density - can be measured simultaneously was used in 160 patients with brain tumors to test the complete potential of magnetic resonance (MR) imaging for quantitative tissue characterization. MR tissue parameters of a wide variety of tumors, edema, white matter, and gray matter were determined. While normal brain tissues showed only small interindividual variations of tissue parameters, pathologic entities (tumor, edema) were characterized by wide ranges of T1, T2, and proton density values. The considerable overlapping of tissue parameters in different tumor entities, which were analyzed in a three-dimensional T1/T2/proton density space, indicates that a reliable diagnosis cannot be made on the basis of a quantitative evaluation of T1, T2, and proton density alone.