The granulocyte chemiluminescence (CL) response is the result of activating its cellular "respiratory burst" and oxidative metabolism. The resulting light emission is an indication of intact metabolic events important in bactericidal activity. Patients with cystic fibrosis have recurrent pulmonary infections. To determine whether granulocytes from patients with CF have defective oxidative metabolism, CL was assayed in 8 patients and compared with that in normal control subjects. In CF, the peak CL response to opsonized zymosan is normal. If, however, the time required for peak light emission is compared with the NIH Clinical Score, a significant correlation is found. Granulocytes from patients with airway disease and more severe CF have a more rapid onset of the CL response. Although the clinical significance of this observation is not established, granulocytes from patients with CF appeared "primed" in their responses to a phagocytic stimulus.