Purpose of our study was to develop a reliable model to define clinical stage I nonseminomatous germ cell tumors (NSGCT) being at low risk and at high risk for occult retroperitoneal metastases based on pathohistological and immunohistochemical parameters in order to stratify the therapeutic approach. 3-5 paraffin-embedded formalin fixed tissue blocks of 149 clinical stage I NSGCT were available from all patients and were analyzed for histopathological features associated with pathological stage: presence/absence of vascular invasion, presence/absence of tunical invasion, percentage of each histological cell type present in the primary tumor. Immunohistochemical expression of MIB-1, p53, bcl-2, cathepsin D and e-cadherin was evaluated using a semiquantitative scoring ystem. Statistical analysis was performed by univariate and multivariate logistic regression models. Percentage of embryonal carcinoma [%EC (p < 0.001)] and presence of vascular invasion [VI (p < 0.0001)] were the most significant independent risk factors associated with pathological stage II disease. Combination of %EC and VI allowed correct prediction of final pathological stage in 88% of patients. Cut-off values including both variables identified correct pathological stage in 131/149 patients (88%). Less than 45% EC and absence of VI correctly identified pathological stage I disease in 91.5%; more than 80% EC and presence of VI correctly predicted pathological stage II in 88% of the patients. %EC and presence/absence of VI appear to be reliable prognosticators to identify both patients at high risk and at low risk for occult retroperitoneal disease. P53, bcl-2, MIB-1, cathepsin D and e-cadherin did not appear to be of prognostic value in clinical stage I NSGCT.