Based on the results of a retrospective study, which found blood vessel invasion to be the most important prognostic factor in clinical stage I nonseminomatous testicular germ cell cancer (NSTGCC I), a prospective study was started in 1985 which assigned NSTGCC I patients without evidence of vascular invasion to surveillance and patients with vascular invasion to two cycles of adjuvant chemotherapy with cisplatin, etoposide, and bleomycin. Twenty-two patients entered the surveillance group and 18 patients received adjuvant chemotherapy. Median follow-up is 30 months (3 to 50 months). Relapses occurred in three patients (7.5%), one in the surveillance group (4.5%), two in the chemotherapy group (11%). Thirty-eight patients (95%) are alive and without evidence of disease. Two patients of the adjuvant-treated group died, one of progressive germ cell cancer and one of lung cancer. We conclude that low- and high-risk NSTGCC I patients can be identified by considering blood vessel invasion. The presence of embryonal carcinoma and vascular invasion seem to be interrelated prognostic factors, because in 94% of vessel invasion the invading element was embryonal carcinoma. The exclusion of patients with vascular invasion from surveillance decreases relapse rates remarkably. Adjuvant chemotherapy diminishes relapse rates in high-risk patients but does not entirely prevent relapse.