The clinical course of 186 orchiectomized patients with testicular non-seminoma clinical stage I (CSI) was reviewed together with a reevaluation of the haematoxylin-eosin-stained histological sections of the primary tumour. Treatment (1970-1980) was as follows: abdominal radiotherapy (group 1): 132 patients; observation only (group 2): 23 patients; retroperitoneal lymph node dissection (group 3): 31 patients. Failure of the management (diagnostic error/relapse after treatment) was defined as demonstration of retroperitoneal lymph node metastases (group 3) or relapse during follow-up (group 2 and 3). Ten of the 31 primarily operated patients had retroperitoneal lymph node metastases. (These patients received 3-4 cycles adjuvant cisplatin based on chemotherapy.) Forty-six patients relapsed after a median time of 6 months (range: 2-113). The 10-year cancer-related survival rates for group 1 and 2 were 85 and 73%, respectively. No cancer-related death occurred in group 3 within the first 5 years. In a univariate analysis the following parameters were significantly (p less than 0.05) correlated with management failure: vascular invasion (blood, lymphatic), the presence of syncytiotrophoblasts, the demonstration of the histological subtype MTU. In a multivariate analysis both lymphatic and blood vessel invasion significantly predicted management failure. In 80% of the non-seminoma patients with CSI and both lymphatic and blood vessels invasion in the primary tumour failure of the primary management must be expected. In the multivariate analysis the demonstration of MTU was a predictive factor only if no differentiation was made between lymphatic and blood vessel invasion. The presence or absence of endodermal sinus tumour was not correlated with the failure rate in non-seminoma CSI. Based on easily assessable histopathological parameters of the primary tumour (small vessel invasion in particular) it is possible to define subgroups of patients with non-seminoma CSI who have a high or a low risk of relapse or who are likely to have microscopic retroperitoneal lymph node metastases. Such a differentiation should have therapeutic consequences.