OBJECTIVES: To analyze whether C-reactive protein (CRP) predicts the need for urgent urinary diversion in patients with renal colic and urolithiasis. CRP may help in the differential diagnosis of complicated hydronephrosis. METHODS: Prospective study done on 110 consecutive patients with renal colic secondary to upper urinary tract calculi admitted in the emergency room. Clinical and analytical data were collected. Criteria for emergency drainage had been established in advance, based on the risk of sepsis, renal failure, persistence of pain, and findings on computed tomography scan. CRP was blindly determined using immunoturbidimetric assay on the Integra 700 analyzer. Statistical analysis included Mann-Whitney test, Cox multivariate analysis, and receiver operating characteristic curves, to determine optimum cut-off points to decide drainage based on laboratory data. RESULTS: Mean CRP value was 47.6 mg/L (CI, 31.4-63.8), 139.6 mg/L (CI, 13-183.1) in 29 patients treated with diversion and 14.67 mg/L (CI, 6.7-22.5) in the control group (P <.001). Age, sex, rate of patients with hypertension, history of cardiovascular disease, leukocyte total count, and serum creatinine differed between groups (P <.05). Regression analysis revealed CRP (P <.0001) and age (P = .0001) were predictive of urinary diversion. Receiver operating characteristic analysis revealed 68.4% area under the curve for creatinine, 68.8% for leukocytosis, and 86.8% for CRP. A cut-off point for CRP of 28 mg/L achieved optimum sensitivity (75.8%) and specificity (88.9%) for determining the decision for drainage. CONCLUSIONS: Determination of CRP in patients with renal colic due to urolithiasis provides an objective and useful parameter for deciding placement of urinary stent, which is even more valuable than leukocytosis or seric creatinine level.