By a microscopic examination of a catheterized specimen of urine, acute appendicitis cannot always be differentiated from acute pyelitis. Given a patient with pain in the right side of the lower abdomen, nausea, vomiting, fever and leukocytosis, the diagnosis will most likely be acute appendicitis. Should a catheterized specimen of this patient's urine contain considerable pus, the condition will probably be acute pyelitis rather than acute appendicitis. For some time I was under the impression that operation should always be withheld in patients with suspected appendicitis and an associated pyuria because of the likelihood of pyelitis being the correct diagnosis. The impression was incorrect, as appendicitis and pyuria may be present at the same time and delaying the operation because of the pyuria may possibly cause a fatal outcome. Such patients should be carefully studied and the decision for or against operative treatment must depend on clinical judgment.