Postoperative serum interleukin-6 (SIL-6) and C-reactive protein (SCRP) levels were examined in 71 patients who underwent various types of abdominal surgery. Similar time-dependent changes in SIL-6 and SCRP levels were observed in 12 patients despite differences in surgical procedures and liver function among the patients. SIL-6 started to increase within 3 hours after the beginning of the operation and reached a peak after 24 hours. SCRP started to increase after 12 hours and was maximum at 48 to 72 hours. The increase in SIL-6 at 24 hours (delta IL-6) showed a close correlation with that of SCRP at 48 hours (delta CRP) in 53 patients without liver cirrhosis. In 18 patients with liver cirrhosis, delta CRP relative to delta IL-6 was less than that in patients without cirrhosis and was poorly correlated with the latter. delta IL-6 was correlated with the length of time of the operation and blood loss in both groups, but delta CRP showed no significant correlation with these factors in either group. These findings indicate that the increase in IL-6 triggered by a surgical procedure may function as a hepatocyte-stimulating factor and that monitoring of SIL-6 may be more helpful than monitoring of SCRP for estimation of inflammatory status and early detection of an acute-phase response.