The value of Magnetic Resonance Imaging (MRI) in the diagnosis, staging and treatment evaluation of retroperitoneal lymph node metastases of non-seminomatous germ cell tumours of the testis (NSGCT) was prospectively studied in 41 consecutive patients before, during and after cisplatin-based chemotherapy. MRI is a non-invasive method, with results equivalent to laparotomy in determining the anatomical localization and size of retroperitoneal lymph node metastases of non-seminomatous germ cell tumours of the testis and residual mass after chemotherapy. Components responding differently can be observed with MRI within the retroperitoneal lymph node metastases on the basis of the tumour signal intensity or homogeneity in the T1- and T2-weighted images; MRI cannot warrant any conclusion about viability of the residual mass or other effects of chemotherapy.