The release of oxygen radicals by blood and bone marrow leukocytes of patients with Fanconi anemia (FA) has been studied. It was found that the nonstimulated FA leukocytes and those stimulated by concanavalin A, SiO2, latex, and opsonized zymosan produced enhanced levels of luminol- and lucigenin-dependent chemiluminescence (CL) in comparison with normal leukocytes. At the same time, the ratio of the intensity of lucigenin-dependent CL to that of luminol-dependent CL was significantly smaller for FA leukocytes than for normal leukocytes. From these findings and from the effects of antioxidative enzymes and free radical scavengers on CL, it was concluded that FA leukocytes release enhanced amounts of oxygen radicals and that these free radicals contain enhanced amounts of hydroxyl or hydroxyl-like radicals more active than superoxide ion. It was proposed that elevated reactivity of the oxygen radicals released by FA leukocytes may be a major factor in the development of Fanconi anemia; this proposal is supported by the first positive results of treatment of FA patients with rutin (a nontoxic natural free radical scavenger and chelator).