To clarify the phathophysiological role of macrophage colony stimulating factor (M-CSF) in the healing processes of inflammation, we studied chronological changes in serum concentrations of M-CSF and other reactants during the acute phase of inflammation in patients who underwent open heart surgery, since these patients showed a precise point of tissue damage. The number of neutrophilic leukocytes increased in the very early postoperative stage, reached a peak 48 h after surgery and declined to the basal level at day 14. The chronological changes in serum C-reactive protein level after surgery were similar to these of neutrophic leukocytes. Serum M-CSF concentrations increased gradually, reached a peak 7 days after surgery and remained at that level until day 14. Based on these phenomena, it is assumed that M-CSF plays a pathophysiological role in the healing processes of an inflammation. Measurement of the serum concentration of M-CSF would be useful for evaluating the process of inflammation.