The aims of this study were to analyze lymphocyte and eosinophil counts in consecutive peripheral blood samples taken during immunotherapy for metastatic renal cell carcinoma (mRCC) and to correlate the findings with objective response and survival. A total of 40 patients with mRCC who received immunotherapy with interleukin-2, interferon-alpha, and 5-fluorouracil were analyzed. Objective responses were observed in 14 patients, including 2 (5%) who showed a complete response (CR) and 12 (30%) who showed a partial response (PR). Eleven patients (27%) achieved stable disease (SD), and 15 patients (38%) had progressive disease (PD). Changes from baseline in the total lymphocyte counts were significantly higher in the responding patients (CR+PR+SD) than in the non-responding patients (PD) (p=0.017), but no difference was seen in the total eosinophil counts (p=0.275). Univariate analysis identified the Eastern Cooperative Oncology Group (ECOG) performance status (p=0.017), the presence of a primary renal tumor (p<0.001) and the peripheral lymphocyte counts at week 4 (p=0.034) as prognostic factors, but a low ECOG performance status (p=0.003) and the presence of a primary renal tumor (p=0.001) were identified as independent poor prognostic factors by multivariate analysis. This study provides further evidence that changes in blood lymphocyte counts may serve as an objective indicator of objective responses.