Blood phagocytes from patients with asthma not treated with antiinflammatory drugs have an increased capacity to produce reactive oxygen metabolites. We studied the phenomenon in a group of patients with asthma receiving inhaled steroids with whole blood chemiluminescence (CL), a simple reactive oxygen metabolite assay suitable for routine use. There was no difference in CL values between patients with stable asthma and no current obstruction (n = 19; mean CL +/- SD = 26.01 +/- 6.51 mV x min) and healthy control subjects (n = 20; mean CL +/- SD = 27.37 +/- 6.15 mV x min), whereas the patients with asthma and current obstruction (n = 27) had significantly higher CL responses (mean CL +/- SD = 41.10 +/- 14.79 mV x min) than both the patients with nonobstructive asthma and the healthy control subjects (p < 0.0005). There was a negative correlation between whole blood CL and the forced expiratory volume in 1 second of the patients with asthma (Spearman's r = -0.47). When the CL values were corrected for the number of phagocytes present in the reactions, the patients with nonobstructive asthma had significantly lower values than the healthy control subjects (p < 0.05). We suggest that the CL values of the patients with nonobstructive asthma reflect the result of effective antiinflammatory treatment and that whole blood CL may be useful as a systemic parameter in the follow-up of the inflammatory process in asthma.