Cisplatin-containing chemotherapy has dramatically improved the outlook for patients with metastatic germ cell tumors (GCT), and overall cure rates now exceed 80%. To make appropriate risk-based decisions about therapy and to facilitate collaborative trials, a simple prognostic factor-based staging classification is required. Collaborative groups from 10 countries provided clinical data on patients with metastatic GCT treated with cisplatin-containing chemotherapy. Multivariate analyses of prognostic factors for progression and survival were performed and models were validated on an independent data set. Data were available on 5,202 patients with nonseminomatous GCT (NSGCT) and 660 patients with seminoma. Median follow-up time was 5 years. For NSGCT the following independent adverse factors were identified: mediastinal primary site; degree of elevation of alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), and lactic dehydrogenase (LDH); and presence of nonpulmonary visceral metastases (NPVM), such as liver, bone, and brain. For seminoma, the predominant adverse feature was the presence of NPVM. Integration of these factors produced the following groupings: good prognosis, comprising 60% of GCT with a 91% (89% to 93%) 5-year survival rate; intermediate prognosis, comprising 26% of GCT with a 79% (75% to 83%) 5-year survival rate; and poor prognosis, comprising 14% of GCT (all with NSGCT) with a 48% (42% to 54%) 5-year survival rate. An easily applicable, clinically based, prognostic classification for GCT has been agreed on between all the major clinical trial groups who are presently active worldwide. This should be used in clinical practice and in the design and reporting of clinical trials to aid international collaboration and understanding.