This study of 93 patients with Stage I nonseminomatous and mixed germ cell testicular tumors who were placed in a surveillance study following orchiectomy was designed to evaluate pathologic prognostic factors. Follow-up was at least 12 months post-orchiectomy except for one patient who was followed for 9 months. Lymphatic invasion was identified in 26 patients, 62% of whom developed distant metastases; metastasis developed in only 18% of 67 patients without lymphatic invasion (P less than 0.01). Relapse was also associated with the presence of embryonal carcinoma. Of 81 patients with an embryonal carcinoma component, 35% developed metastases, whereas none of those without an embryonal carcinoma developed metastasis (P = 0.05). Effects of other histologic features and tumor size were not significant. Lymphatic invasion appeared to be a significant poor prognostic factor, and embryonal carcinoma was an independent poor prognostic factor.