The aims of the present study were to: (i) develop a clinically useful prognostic classification in Asian patients with metastatic renal cell carcinoma (RCC) by combining metastatic features with several pretreatment parameters; and (ii) evaluate the validity of this prognostic classification. Baseline characteristics and outcomes were collected for 361 patients who underwent interferon-a-based therapy between 1995 and 2005. Relationships between overall survival (OS) and potential prognostic factors were assessed using Cox's proportional hazard model. The predictive performance of the model was evaluated using bootstrap resampling procedures and by using an independent dataset obtained from randomly selected institutions. The predictive accuracy was measured using the concordance index (c-index). Four factors were identified as independent prognostic factors: time from initial diagnosis to treatment, anemia, elevated lactate dehydrogenase (LDH), and poor prognostic metastatic group (liver only, bone only, or multiple organ metastases). Each patient was assigned to one of three risk groups: favorable risk (none or one factor; n = 120), in which median OS was 51 months; intermediate risk (two factors; n = 101), in which median OS was 21 months; and poor risk (three or four factors; n = 102), in which median OS was 10 months. The c-index was 0.72 in the original dataset and 0.72 in 500 random bootstrap samples. In the independent dataset for external validation, the c-index was 0.73. Thus, the new prognostic classification is easily applicable for Asian patients with previously untreated metastatic RCC and should be incorporated into patient care, as well as clinical trials performed in Asia. (Cancer Sci 2012; 103: 1695-1700 W ith the advent of molecular targeted therapy, treatment for metastatic renal cell carcinoma (RCC) has changed markedly. At present, drugs that should be used for molecular targeted therapy are usually selected on the basis of the Memorial Sloan-Kettering Cancer Center (MSKCC) risk classification.(1,2) This results from the fact that clinical trials of molecular targeted drugs, such as sunitinib and temsirolimus, have been performed by selecting subjects on the basis of this risk classification and their usefulness has been clarified. (3,4) In Japan, the MSKCC risk classification is widely used not only in clinical trials, but also in daily clinical practice.(5,6) However, whether this risk classification is useful for predicting outcomes in Japanese patients with metastatic RCC is not unclear. Recently, we performed a retrospective study on the MSKCC risk classification in patients with metastatic RCC and reported its applicability to Japanese patients.(7) However, when this risk classification was used in Japanese RCC patients, the percentage of patients in the poor risk group was high compared with that in Western series and the survival period of the poor risk group was twice as long in Japanese patients as that in patients in Western countries. (7,8) This may be because multiple organ me...