Serum B2m concentrations were evaluated preoperatively in 40 patients with renal cell carcinoma and normal renal function, as assessed by serum creatinine less than 1.4 mg/dl, and compared with those of 23 age-matched controls. Mean value +/- SD was 3,088 +/- 966 ng/ml for renal cancer patients, while controls had a value of 1,800 +/- 240 ng/ml. Statistical analysis, performed by Student t test, revealed a very high degree of significance (p less than 0.0005). No statistically significant differences were found between groups of patients classified according to tumor stage and cell type. Seventy percent of renal cell carcinoma cases had preoperatively elevated serum levels of B2m.