T lymphocyte subsets and lymphocyte activation markers in the peripheral blood were assessed in patients admitted to hospital with acute severe asthma (status asthmaticus). Measurements were made on admission, day 3, and day 7 (or on discharge from hospital if this was sooner). The results were compared with 3 control groups (mild asthma, chronic obstructive airways disease, and normal individuals). The percentages of CD4-positive and CD8-positive T lymphocytes, and the CD4/CD8 ratios, were similar in the patients with acute severe asthma and the control groups, and were within the normal range. In contrast, patients with acute severe asthma had significant increases, compared with control subjects, of three surface proteins associated with T lymphocyte activation: interleukin-2 receptor (IL-2R); class II histocompatibility antigen (HLA-DR); and "very late activation" antigen (VLA-1). The IL-2R-positive T lymphocytes were exclusively of the CD4 "helper-inducer" phenotype. The percentages of IL-2R-positive and HLA-DR-positive (but not VLA-1-positive) lymphocytes tended to decrease as the patients were treated and clinically improved, although these values remained raised above control values for the observation period. Cell-mediated immunity may be causally related to the pathogenesis of acute severe asthma.