The effect of 6 putative prognostic factors on survival was studied in patients with Stages III and IV malignant teratoma of the testis. Differences between survival curves were tested for statistical significance. A diameter greater than 5 cm in the largest tumour mass, and greater than 8 pulmonary metastases were adverse prognostic factors (P = 0.004 and 0.008 respectively). Patients with malignant teratoma, trophoblastic, fared worse than those with malignant teratoma, undifferentiated, and malignant teratoma, intermediate (P = 0.011 and 0.023 respectively). Previous chemotherapy or radiotherapy had no significant effect. Serum alpha-foetoprotein (AFP) above 10(3) MRC u/ml and serum beta subunit of human chorionic gonadotrophin (hCG) above 10(5) miu/ml, were found to predict a poor prognosis (P = 0.010 and 0.001 respectively). A combination of measurements of the tumour markers gave the most consistent indication of prognosis, in that patients with either AFP greater than 10(3) MRC u/ml or hCG greater than 10(5) miu/ml, or both, fared much worse than those with neither factor (P = 0.001). Serum concentrations of AFP and hCG should be stated in reports of treatment of testicular teratoma in order to provide a basis for comparison with other series. Regular and frequent measurements of these markers are appropriate throughout the clinical management of patients with malignant teratoma.