We evaluated whether serum soluble interleukin 2 receptor (sIL-2R), a marker of T lymphocyte activation in vivo, could be useful to monitor disease activity in asthma. Venous blood was collected from 26 patients with acute severe asthma prior to the commencement of systemic corticosteroid therapy (day 1), 15 with stable disease, and 13 normal control subjects. Serum sIL-2R level was significantly higher in acute asthma (462.7 +/- 36.1 U/ml; mean +/- SEM) than stable disease (328.5 +/- 30.4 U/ml, p = 0.013) which in turn, was significantly raised when compared with control subjects (239.0 +/- 22.9 U/ml; p = 0.0003 vs acute; p = 0.036 vs stable). Nevertheless, sIL-2R concentrations in 11 patients with acute and 11 with stable disease did not exceed the upper limit of normal, ie, mean +.2 SD of the value in control subjects = 404.4 U/ml. Repeated measurements of sIL-2R in 24 acute asthmatics on day 3 revealed no significant fall (464.6 +/- 37.2 U/ml, NS), although the reduction in sIL-2R was significantly correlated with the corresponding improvement in peak expiratory flow (r = -0.52, p = 0.005). Following resolution of the acute attack, further measurements performed in 11 of these subjects on day 28 showed a significant fall in sIL-2R (p = 0.016). Our data showed that although serum sIL-2R was raised in asthma and, to a certain extent, might reflect disease activity, the considerable overlap of values between asthma of differing severity and normal control subjects precludes its clinical use as an index of asthma severity.