In this prospective study of 442 infants younger than 8 weeks of age who attended a pediatric emergency department with temperature greater than or equal to 100.6 degrees F (38.1 degrees C), urinary tract infections (UTIs) were found in 33 patients (7.5%), 2 of whom were bacteremic. Clinical and laboratory data were not helpful for identifying UTIs. Of the 33 patients with UTIs, 32 had urinalyses recorded; 16 were suggestive of a UTI (more than five white blood cells per high–power field or any bacteria present). Of the 16 infants with apparently normal urinalysis results, three had an emergency department diagnosis suggesting an alternative bacterial focus of infection. If the physician had decided on the basis of apparently normal urinalysis results to forgo obtaining a urine culture, more than half of the UTIs would have been missed. Bag–collected specimens were significantly more likely to yield indeterminate urine culture results than either catheter or suprapubic specimens. In addition, uncircumcised males were significantly more likely to have a UTI than circumcised boys. These results suggest that a suprapubic or catheter–obtained urine specimen for culture is a necessary part of the evaluation of all febrile infants younger than 8 weeks of age, regardless of the urinalysis findings or another focus of presumed bacterial infection.