Objectives: To analyze the clinical value of C-reactive protein (CRP) as a marker of prolonged hospitalization and a predictor of recurrence in patients after uncomplicated acute pyelonephritis (APN). Methods: A total of 202 consecutive adult patients with APN were prospectively enrolled from September 2005 to June 2007. APN was defined as the concomitant presence of 4 major and ≥2 minor clinical or laboratory signs or symptoms suggestive of APN. All patients were treated with parenteral antibiotics. The patients were discharged after normalization of body temperature, serum white blood cell counts, and urinalysis. Correlations among the recurrence of APN and various factors, including CRP, were investigated. Results: Of the 202 patients, 13 were excluded because of the presence of complicating factors or insufficient data. APN recurrence developed in 4 patients (2.1%). The CRP level at discharge correlated significantly with the recurrence of APN on univariate and multivariate analysis. Irrespective of the normalization of body temperature, serum white blood cell counts, and urinalysis, the recurrence of APN was significantly greater in the patients with CRP >4 mg/dL than in those with <4 mg/dL at discharge. Patients with a maximal CRP of >15 mg/dL during admission had a longer hospitalization and required more intravenous antibiotic therapy than did the patients with a maximal CRP of <15 mg/dL. Conclusions: As a marker of prolonged hospitalization and recurrence, CRP can complement the present clinical and laboratory parameters used as guides in the proper treatment of patients with uncomplicated APN.