Clinical details of 85 men presenting with previously untreated metastatic seminoma are presented. In Stage II disease relapse rate was related to the size of metastases. In IIA (32 patients) the relapse rate was 9.4%; IIB (11 patients), 18.2%; and IIC (23 patients), 39.1%. The continuous disease-free survival rate was significantly worse for IIC than IIA and IIB patients (P = 0.023). No instance of first relapse in supradiaphragmatic nodes was observed in 13 men with Stage II disease treated with irradiation limited to infradiaphragmatic nodes. In relapsing Stage IIC patients, extralymphatic metastasis was as frequent as abdominal relapse. On the basis of these observations, together with preliminary data in nine men receiving Cis-platinum-containing chemotherapy, all of whom are in complete remission, it is proposed that patients with Stage IIA and IIB disease should receive infradiaphragmatic irradiation with chemotherapy deferred until relapse. Stage IIC patients should receive chemotherapy initially, followed by irradiation. In Stage III and IV disease chemotherapy should be initial therapy with radiotherapy for bulky disease on an individualised basis. Moderate elevation of blood B-HCG levels is not inconsistent with a diagnosis of pure seminoma and does not appear to influence adversely the outcome of radiotherapy.