We studied epinephrine-induced changes in leukocyte counts in twelve persons with acute asthma and nine healthy, nonasthmatic individuals. Twenty-five minutes after subcutaneous injection of 0.3 mg of epinephrine, a significant increase in the mean numbers of total leukocytes was observed in both the asthmatic and control subjects, 9,195 to 10,519 and 6,733 to 9,222, respectively. Compared with controls, asthmatic individuals had increased numbers of total leukocytes, segmented neutrophils, and eosinophils prior to epinephrine, and the elevations in segmented neutrophils and eosinophils persisted after epinephrine. The increase in total leukocytes was largely secondary to an increase in lymphocytes. Lymphocyte increases were greater in control than in asthmatic individuals (P less than .001). Of clinical importance is the finding that leukocyte counts drawn shortly after treatment with epinephrine do not suggest infection in patients whose pre-epinephrine counts were normal.