The aim of this study was to compare diagnostic performance of C-reactive protein (CRP) and poly-C avid ribonuclease (P-RNase) levels in the prediction of a severe clinical course of acute pancreatitis (AP). The study included 36 patients with mild and 20 with severe AP. CRP concentration was measured by an immunonephelometric method and P-RNase activity by the rate of polycytidylate hydrolysis at pH 7.8. At the time of admission, both P-RNase and CRP levels were significantly increased in all patients when compared to healthy subjects (29.2 vs. 18.7 U/l and 91.1 vs. 2.89 mg/l; p < 0.001). Up to days 3 and 4 a further increase in P-RNase was observed. On the other hand, the increase in CRP continued only through days 2 and 3 (p < 0.001). Severe acute pancreatitis (SAP) and mild acute pancreatitis (MAP) differed significantly with respect to P-RNase levels on all days studied; whereas CRP levels differed significantly on days 2-5 but did not differ at admission. Receiver operating characteristic (ROC) curve function analysis yielded the best sensitivity of SAP detection for P-RNase, equaling 72.2%, at the cut-off point value 65.3 U/l on day 3 after admission. The sensitivity of CRP for detection of SAP was 85.0% at 125.7 mg/l on the 2nd day after admission. Both parameters studied were significantly associated with the severity of the AP clinical course; however, on days 1 and 2 post-admission, P-RNase was more specific for detection of SAP than CRP (94.4% vs. 77.1% on the 1st day and 94.4% vs. 55.5% on the 2nd day). In conclusion, P-RNase has shown an excellent performance for early differentiation of acute necrotizing pancreatitis.