OX40 (CD134) is a member of the TNF receptor family and is expressed selectively on activated T lymphocytes. Through interactions with OX40 ligand, OX40 delivers potent costimulatory signals to T cells. Here, we describe a sandwich ELISA method for the detection and quantification of OX40 using the monoclonal anti-OX40 antibodies ACT35 and L106 as capture and detecting antibodies, respectively. With this ELISA, the existence of naturally occurring soluble forms of OX40 (sOX40) could be demonstrated for the first time. Soluble OX40 is detectable in serum of subpopulations of healthy donors and patients with autoimmune disease and cancer. Among the five diseases that were investigated, chronic lymphocytic leukemia was identified as the one with the highest frequency of sOX40-positive sera and with the highest mean sOX40 serum concentration. The stability of sOX40 is not affected by repeated freeze/thaw cycles nor by prolonged storage at -20 degrees C, allowing studies on the potential diagnostic significance of sOX40 using existing banks of sera.