OBJECTIVE: To evaluate the NycoCard:::CRP test (Nycomed A/S, Oslo, Norway) in relation to ESR in consecutive patients in general practice. NycoCard:::CRP test was also compared with a reference method for CRP quantitation. DESIGN: C-reactive protein and ESR were simultaneously measured in 607 consecutive patients at four community health centres. SETTING: Four community health centres in southern Sweden. RESULTS: We obtained consistent results in 71% of the cases. In 20% CRP was increased more than ESR, while ESR was increased more than CRP in 9%. CRP was increased in 16% while ESR was below the upper reference limit for age and sex. On the other hand ESR was increased while CRP was below 10 mg/l in five % of the patients. In most of the cases where there was a discrepancy- for example, in infectious diseases-the CRP results were more clear-cut. Using the NycoCard test the CRP concentration can be measured directly in a whole blood sample with the result available within minutes. Comparison of the NycoCard:::CRP test with the reference method for CRP quantitation showed good agreement. CONCLUSION: In clinical situations with suspected inflammatory diseases, the CRP test appears often to yield more useful results than the ESR. The NycoCard:::CRP test is suitable for use in general practice.