The MRI signal enhancement in a breast tumor, measured as a function of time after a bolus injection of Gd-DTPA, may contain enough information to differentiate malignant from benign tissue. We find a physiological model for measuring capillary permeability and leakage space (P. S. Tofts, A. G. Kermode, measurement of the blood-brain barrier permeability and leakage space using dynamic MR imaging. 1. Fundamental concepts. Magn. Reson. Med. 17, 357-367 (1991)) fits the data well. The enhancement curve is particularly sensitive to the preinjection T1 of the tumor, the dose, and the time of injection. This model may provide a means of characterizing the pathophysiology of breast tumors from the Gd-DTPA enhancement curve.