The role of Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) in the treatment evaluation of retroperitoneal lymph-node metastases of non-seminomatous testicular tumors (NSTT) was prospectively studied in 10 consecutive patients before, during and after chemotherapy. The results thus obtained were compared with laparotomy findings before and after chemotherapy, the histology of the primary testicular tumor, and that of retroperitoneal residual lesions after chemotherapy. MRI and CT proved to be equivalent in detection and in determining the anatomical localization and size of the retroperitoneal lymph node metastases. Unlike CT, MRI revealed unmistakable changes in the structure of the retroperitoneal lymph-node metastases during chemotherapy, for which no histological cause was found except in mature teratoma. In mature teratoma a high T2 signal was found within the metastases, corresponding with a high water content. On the basis of tumor consistency and signal intensity in the T1- and T2-weighted images, MRI cannot yet warrant any conclusion about the ultimate effect of chemotherapy.