Experimental studies have shown that interleukin-6 induces all major acute-phase proteins in the liver, including C-reactive protein. In 50 patients with acute pancreatitis, the serum concentrations of interleukin-6 and C-reactive protein were determined daily during the first week of hospitalization. Patients were divided into three groups according to clinical criteria: mild pancreatitis (<- 1 complication; n = 25), severe pancreatitis (≥ 2 complications; n = 15), and lethal outcome (n = 10). Patients with mild disease showed initially slightly elevated levels of interleukin-6 (22.0 ± 9.8 U/mL) that decreased to low levels within 4 days (5.0 ± 1.0 U/mL). In patients with severe pancreatitis, serum concentrations of interleukin-6 were initially clearly elevated (35.0 ± 7.5 U/mL) and remained slightly elevated until day 7 (13.0 ± 2.0 U/mL). Patients with lethal outcome had markedly elevated initial interleukin-6 concentrations (61.0 ± 15.0 U/mL) that decreased but were still elevated at day 7 (26.0 ± 2.5 U/mL). In all three groups, C-reactive protein concentrations followed the course of interleukin-6 concentrations by 1 day. There was a positive correlation between maximal interleukin 6 concentrations and maximal increases in the serum concentrations of C-reactive protein (r = 0.66). At days 1 and 2, increased (>15 U/mL) interleukin-6 concentrations (positive predictive value, 91%; negative predictive value, 82%) predicted a severe or lethal course of the disease more accurately than elevated [>; 0.10 g/L (>10 mg/dL)] C-reactive protein concentrations (positive predictive value, 67%; negative predictive value, 79%). In conclusion, elevated serum concentrations of interleukin-6 followed by increased levels of C-reactive protein reflect the severity of acute pancreatitis.