To assess prognostic factors for extratesticular involvement in patients with testicular stage I non seminomatous germ cell tumors, we retrospectively studied the clinical and histological features of 58 patients treated at Institut Gustave-Roussy. Twenty-four patients were followed after orchiectomy alone, and 34 patients underwent an initial subsequent retroperitoneal lymphadenectomy. Nineteen patients (33%) experienced an extratesticular involvement after a mean follow-up period of 40 months. All relapses occurred during the first year of follow-up. Disease-free survival at one year was 64% and overall survival was 100%. The clinical and histological features studied were: age, AFP and HCG serum levels before orchiectomy, tumor size, TNM stage, histologic type with the corresponding percentage, venous and lymphatic involvement. The detection of metastasis at lymphadenectomy or during the follow-up after orchiectomy was the single criteria for prognostic factor research. Venous and/or lymphatic involvement as well as a high percentage of embryonal carcinoma component turned out to be the two independent significant prognostic factors which predict extratesticular involvement.