Hairy cell leukemia is a chronic lymphoproliferative disorder of B-cell lineage. Malignant cells express the interleukin-2 receptor (IL-2R) which is released in vitro as well as in vivo. The sera of patients with hairy cell leukemia contain elevated levels of this soluble receptor (sIL-2R). Sera of 24 patients with hairy cell leukemia were tested for sIL-2R. In 9 patients treated with 2-chlorodeoxyadenosine an improved clinical status was associated with decreasing serum sIL-2R. The maximal rate of decrease of sIL-2R level was observed within the second and the third week after the therapy initiation. Patients with disease progression had an increase in serum sIL-2R level. Our results suggest that serial measurement of sIL-2R level can be used as a reliable, noninvasive means to assess the disease activity and its response to therapy.