Background: Previous studies have reported that soluble (s) CD86 is involved in the initiation of the immune response. A study was undertaken to investigate the concentrations of sCD86 in serum samples from patients with bronchial asthma and to determine the cell origin of sCD86. Methods: Serum sCD86 concentrations were measured in 52 asthmatic subjects and 25 non-atopic normal volunteers using an enzyme linked immunosorbent assay, and the relationship of serum sCD86 concentrations to asthma severity and to total and differential white cell counts was analysed. Each type of white blood cell was purified and cultured in vitro to determine the cell origin of serum sCD86. Results: Serum samples from patients with an acute asthma exacerbation had much higher levels of sCD86 (585.4 (20.5)IU/ml) than those from stable asthmatics (479.6 (15.7)IU/ml, p,0.001) and healthy individuals (435.1 (13.8)IU/ml, p,0.001), and there was no difference between the latter two groups (p=0.079). In asthmatic subjects the serum sCD86 level was inversely correlated with airway responsiveness, forced expiratory volume in 1 second, and with arterial carbon dioxide tension. In addition, the serum sCD86 level was positively correlated with numbers of lymphocytes, eosinophils, monocytes, but not neutrophils. The in vitro experiments indicated that sCD86 was produced by monocytes. Conclusions: The serum sCD86 protein level was significantly increased in asthmatic subjects during an exacerbation and correlated with the severity of asthma. sCD86 is most probably derived from monocytes in the peripheral blood.