The United Kingdom Children's Cancer Study Group (UKCCSG) malignant germ cell tumour (MGCT) studies were undertaken to establish standard protocols of investigation, staging, and treatment. The efficacy of new drug combinations and the value of serial measurements of serum alphafetoprotein (AFP) and human chorionic gonadotrophin (HCG) were evaluated. Following the initial surgery, staging of the tumour was performed using a variety of investigative approaches. In stage 1 testicular tumours, orchidectomy was performed. In more advanced tumours, and in stage 1 tumours that failed to show the expected decline in AFP or recurred, chemotherapy was used after appropriate surgery. Seventy-three boys, under 14 years of age, with testicular MGCTs have been entered into the UKCCSG studies since 1979. Serum AFP was measured preoperatively, or within 2 weeks of operation, in 70 boys. It was unequivocally elevated in 69. Monitoring by serial AFP measurement proved valuable in assessing response and in early detection of recurrence. HCG was measured in 46 boys, and was raised in three. Sixty-seven (91%) of the tumours were yolk sac (Teilum) tumours, four were immature teratoma, and two were mixed MGCTs. The only non-AFP producing tumour was an immature polydermal teratoma in a 1-year-old boy. Serum HCG was raised in three boys with yolk sac tumours, one with a mixed teratoma, and one 14-year-old boy who had a mixed MGCT. The results of treatment were assessed on April 1, 1989 (median time from diagnosis, 3 years 4 months). Seventy-one boys were alive, 48 of whom had been cured by orchidectomy alone. The remaining 25 patients received chemotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)