The finding of eosinophils in the urine has been suggested to be useful in establishing the diagnosis of acute interstitial nephritis (AIN). The diagnostic accuracy of this test has not yet been defined. It is the purpose of this study to define the specificity, sensitivity, and the predictive positive and negative values for the presence of eosinophils in the urine. One hundred forty-eight patients with pyuria were tested for the presence or absence of urinary eosinophils. In this group consecutively admitted to the hospital with WBC in the urine, 4% of patients had urinary eosinophilia of greater than 1 eosinophil per 100 cells. Since none of this group had the diagnosis of AIN, the false positive rate was 4% and the specificity was 96%. In a selected group of patients in which the diagnosis of AIN was suspected by a nephrology consultant, urinary eosinophils were found in 6 of 15 patients with a confirmed diagnosis of AIN but were also found in 10 of 36 patients with another renal diagnosis. The sensitivity for eosinophiluria was 40% and the specificity was 72% with a positive predictive value of only 38%. We conclude that eosinophiluria is not an accurate test for the diagnosis of AIN. The false positive and negative rates are too high to confirm an AIN diagnosis.