Anastomotic leakage (AL) is a severe complication following colorectal surgery. C-reactive protein (CRP) is considered to be an indicator of postoperative complications. Between August 2002 and August 2005 342 colorectal resections with primary anastomosis were performed at the Department of General and Vascular Surgery. Johann Wolfgang Goethe-University Frankfurt. For this retrospective study serum CRP was measured daily until postoperative day 7, and in cases of AL it was excluded from statistical analysis beginning with the day on which the AL was diagnosed. Twenty-six of 342 (7.6%) patients developed AL at a mean of 8.7 days postoperatively. The in-hospital mortality was 3.5% for all patients and was significantly higher in the AL group (11.5 versus 2.8%). The CRP level in the two groups showed a peak on day 2.5 and day 2.2, respectively. In case of postoperative AL the CRP level did not show a marked decline during the next few days. Compared to the cases where AL did not develop, there was a significantly higher increase in CRP from the preoperative level to the levels measured on postoperative day 3, 5, 6 and 7. Higher CRP levels were observed in patients experiencing pneumonia or urinary tract infection, but the decrease of CRP values was not as slow as in cases of AL. This study shows serum CRP level to be a relevant marker in detecting postoperative complications in colorectal surgery. Prolonged elevation and a missing decline in CRP level precede the occurrence of AL.