Noninvasive tomochemical analysis of normal and pathological brain tissues of seven patients has been performed with the technique of dual energy computed tomography. The variation of the effective atomic numbers observed in this experiment is relatively small, less than 6%, while the variation of the electron density is below 4%. Cerebral pathology involves complex variations of molecular constituents, even in macroscopically apparently simple lesions like edema, tumors, hemorrhage, etc. These molecular variations may have opposing, equalizing, or additive effects on the effective atomic number. Our results, supported by existing in vitro data, suggest that the clinical value of this technique is limited.