Sera from 70 patients with bladder cancer, 26 patients with kidney cancer, and 20 patients with prostate cancer were studied for the presence of circulating immune complexes (CIC) utilizing the human red blood cell (HRBC) radioimmunoassay. Elevated CIC levels were found in 32% of patients with bladder cancer, 30% of patients with kidney cancer, and 20% of patients with prostate cancer. For comparison, CIC was also measured in 37 sera from patients with bladder cancer by the polyethylene glycol precipitation complement consumption (PEG-CC) test. 13 of them (35%) had elevated CIC levels. The correlation between the results of the two tests was significant. There were significant correlations between CIC levels and the extent of tumor invasion and the degree of malignancy in patients with bladder cancer. The elevation of CIC level was also significantly associated with poor prognosis of these bladder cancer patients. It is proposed that measurement of CIC in patients with bladder cancer by the HRBC radioimmunoassay may be useful to evaluate their stage, degree of malignancy, and prognosis.