Serum C-reactive protein levels and erythrocyte sedimentation rates were prospectively evaluated in 38 patients with dilated pyelocaliceal systems in an attempt to distinguish simple hydronephrosis from infected hydronephrosis and pyonephrosis. The clinical presentations of infected hydronephrosis and pyonephrosis were extremely variable, ranging from no constitutional complaints to urosepsis. Renal sonography detected only 6 of 16 patients with pyonephrosis (specificity 96%, sensitivity 38%, accuracy 72.5%). Using a cutoff value of 3.0 mg./dl. for C-reactive protein and 100 mm. per hour for erythrocyte sedimentation rate, the diagnostic accuracy of detecting infected hydronephrosis and pyonephrosis increased to 97%, with a specificity of 89% and sensitivity of 100%. Based on our experience, we believe that the serum C-reactive protein and erythrocyte sedimentation rate levels can be used as screening tests to distinguish pyonephrosis and infected hydronephrosis from simple, uncomplicated hydronephrosis.