Hairy cell leukemia is a chronic lymphoproliferative disorder of B-cell lineage, whose malignant cells express the interleukin-2 (IL-2) receptor. A soluble form of the IL-2 receptor is released by these cells in culture, and the sera of patients with hairy cell leukemia contain elevated levels of this soluble receptor. Four hundred twenty- seven serum samples from 101 patients were analyzed for soluble IL-2 receptor (sIL-2R). The clinical status of patients appeared to be associated with the serum level of sIL-2R. The hairy cell index (a measure of tumor cell burden) was correlated with the square root of the serum sIL-2R level (r = .77). Improved clinical status was associated with decreasing serum sIL-2R levels, whereas disease relapse was associated with increasing levels. Notably, every patient who responded to therapy had a decline in serum sIL-2R level, and every patient with disease progression had an increase in serum sIL-2R level. This phenomenon was observed for several different treatments, including standard-dose interferon, low-dose interferon, and deoxycoformycin. The predictive reliability of this test is currently being prospectively evaluated.