The phenotypic distribution and immune reactivity of T lymphocyte subpopulations from peripheral blood of 50 patients with urological cancer were determined. Included were 36 patients with bladder transitional cell carcinoma, 7 patients with renal cell carcinoma and 7 patients with prostatic carcinoma. Thirty-eight age-matched patients with benign urological disease served as controls. A depression in immune competence was found in the group of male patients with infiltrating bladder cancer. In more than 50% of the patients with infiltrating bladder carcinoma, the T helper (CD4) subset was reduced with a concomitant inversion in the CD4/CD8 ratio and impairment in the T cell function as determined by the ability to proliferate upon phytohemagglutinin and concanavalin stimulation. Patients with superficial bladder carcinoma, as well as those with renal cell carcinoma had an immune profile similar to that of the control group. The group of patients with prostatic carcinoma had higher mean CD4/CD8 ratios than the control group, resulting from decreased suppressor/cytotoxic cells. Our results have indicated that the characterization of T cell subset and lymphocyte activity correlated well with the histopathologic state of patients with bladder carcinoma. Thus, the determination of the CD4/CD8 ratio may prove a valuable method for monitoring patients with bladder carcinoma, in addition to serial urine cytology, random urothelial biopsies and flow cytometry.