We measured the serum concentration of soluble interleukin-2 receptor (sIL-2R) in asthma patients and healthy controls to evaluate T-lymphocyte activation in this disease. The SIL-2R concentrations in patients with asthma irrespective of presence of acute attacks were significantly higher than those of the controls (p < 0.001). Although the sIL-2R concentrations were similar in the patients with acute asthma and those in remission, levels in patients with moderate and severe asthma attacks were significantly higher than levels for patients in remission (p < 0.001). These results suggest that T-lymphocyte activation occurs in asthma patients and becomes more prominent during moderate and severe acute attacks.