Peripheral blood lymphocytes and leukocyte levels were monitored in 34 patients with bladder carcinoma before, during, and up to 5 years after radiotherapy. Radiotherapy in doses 6500 to 8500 rads caused a marked decline in the numbers of circulating leukocytes and particularly lymphocytes. In patients clinically free of disease for 5 years, lymphocyte counts returned to pretherapy levels within 3 years after radiotherapy. In contrast, in patients with recurrent or residual tumors lymphocyte counts failed to reach pretherapy levels within 3 years after therapy. The rate of recovery from radiation-induced lymphopenia was significantly different for patients who were free of disease as compared to those with recurrent or residual tumor (p less than 0.05). No correlation was found between posttherapy leukocyte levels and clinical status.