Urinary tract infection especially pyelonephritis is a common condition worldwide caused by different uropathogens. The hostresponse parameters like C-reactive protein (CRP) are important inflammatory markers and diagnostic tests. The aim of this study was to assess the usefulness of CRP in urinary tract infection-pyelonephritis caused by different bacteria for defining the urinary tract infection (UTI). A total of fifty patients with clinical symptoms of pyelonephritis referred to Al-Kindi Teaching Hospital (Baghdad, Iraq), were investigated from October 2009 to November 2009. Clean-catch midstream urine of the patients was collected. Urine specimens were cultured for isolation of the microbial agents of UTI. The isolated bacteria were identified using biochemical tests. CRP were assessed in relation to the inflammatory responses. The most common isolated bacteria were (40%), Enterobacter (20%) and Klebsiella (12%). The least frequent bacteria were (4%), (4%) and (4%). There was a significant difference (P<0.001) between systemic inflammatory responses (CRP) with urinary tract inflammatory response (pyuria). According to the findings of this study, CRP can be suggested as useful and valuable tests in pyelonephritis.