Chemiluminescence response upon phagocytosis was found to be enhanced in circulating blood monocytes derived from patients with active sarcoidosis, suggesting that these monocytes are activated and easily generate reactive species of oxygen including singlet oxygen. Such enhancement was observed even when patients were receiving high doses of corticosteroids but not when the sarcoidosis was chronic or the patients had recovered. Chemiluminescence measurements may thus be a useful aid for confirming diagnosis and assessing the effects of treatment of the disease.