259 patients with stage I non-seminomatous germ-cell testicular teratoma who were treated by orchidectomy alone and monitored at one often centres in the United Kingdom were followed for a median of 30 months. 62 of the 70 relapses occurred in the first 18 months after orchidectomy. The 2-year relapse-free rate was 74%, falling to 68% at 4 years. Histological sections from 233 of the orchidectomy specimens were reviewed centrally. Four features independently predicted relapses: invasion of testicular veins, invasion of testicular lymphatics, absence of yolk-sac elements, and presence of undifferentiated tumour. An index, based on the number of these features observed, identified a high-risk subgroup of 55 patients who had a 42% relapse-free rate at 2 years.