CD22 is an important immunotherapeutic target on B-cell malignancies, particularly hairy cell leukemia (HCL), but its soluble extracellular domain, sCD22, has not yet been reported in the blood. By immunoaffinity and enzyme-linked immunosorbent assay techniques using anti-CD22 monoclonal antibodies, we identified the 100-kDa extracellular domain of CD22 and an 80-kDa processed form in serum of patients with HCL. The median sCD22 level measured by enzyme-linked immunosorbent assay was 18 ng/mL for 93 patients with HCL. sCD22 levels varied from 2.1 to 163 ng/mL and were higher (P < .001) than 23 normal donors (median, 0.6 ng/mL). More than 95% of normal donors had sCD22 levels less than 1.9 ng/mL. sCD22 levels were proportional to concentrations of circulating HCL cells (P = .002), and HCL spleen size (P < .001). sCD22 levels normalized with complete but not partial response to treatment. sCD22 levels up to 300 ng/mL had less than a 2-fold effect on the cytotoxicity of the anti-CD22 recombinant immunotoxin BL22. sCD22 levels may be useful to follow in patients with HCL and may be more specific than sCD25 in patients with CD22(+)/CD25(-) disease. Trials are listed on www.cancer.gov as NCT00002765, NCT00021983, NCT00074048, NCT00085085, NCT00337311, and NCT00462189.