Surgical specimens from 22 patients with infiltrating ductal carcinoma of the breast were analyzed by high-resolution 1H NMR spectroscopy at 80 and 300 MHz. Results were compared with those obtained from 9 benign (proliferative dysplasia without atypia) breast samples. NMR spectra at 80 MHz showed well-resolved water and lipid peaks so it was possible to calculate specific T1 and T2 relaxation times. The longitudinal T1 relaxation process of the water was found to be properly represented by a monoexponential function, whereas the transverse T2 decay was well accounted for by a biexponential model. Relaxation time measurements of the water were found to be a valuable tool in breast tissue characterization, in particular (a) mean T1 values from malignant and control populations are well distinguished (P less than 0.0008); (b) mean T2 values of both fast and slow components from the two populations are statistically different (P less than 0.0001) and no overlap exists in the distributions of the second component. Three-hundred-megahertz spectra allowed a detailed analysis of the lipidic component of the sample: malignant tumors were found to present in the region 1.3-1.2 ppm a species characterized by a very slow transverse relaxation time (T2 = 200-1000 ms).