This study was performed to investigate the hematologic changes and its pathogenesis in acute carbon monoxide (CO) intoxication. Serial complete blood counts (CBC) were obtained in 17 patients with acute CO intoxication five times in two weeks. Peripheral blood smear, bone marrow aspiration and biopsy were obtained in 7 patients within the first week. We analyzed the results of serial CBC's. Levels of hemoglobin and hematocrit rose only initially, probably due to dehydration and hemoconcentration rather than hypoxia. Leukocytosis, mainly neutrophilic, observed during the first few days seemed to be a physiological phenomena due to a stressful situation, such as hypoxia. Levels of platelet increased steadily after a initial decrease. We found no specific findings in bone marrow.