I wish in this paper to make three points: 1. A goodly number of "classic" time-honored mistakes in diagnosis are familiar to all experienced physicians because we make them again and again. Some of these we can avoid; others are almost inevitable, but all should be borne in mind and marked on medical maps by a danger-signal of some kind: "In this vicinity look out for hidden rocks," or "Dangerous turn here, run slow." I shall enumerate some of these danger points presently. 2. Some common diseases are relatively inaccessible to diagnosis, no matter how carefully we are on the watch for them. From the study of 3,000 autopsies, I have begun to work out a percentage or ratio of accessibility for the commoner diseases (shown in the accompanying chart). 3. Besides the classic and well-known pitfalls there are some less familiar to the profession and needing all the more, therefore, to be marked "dangerous." FREQUENT AND WELL-RECOGNIZED PITFALLS I shall first enumerate without discussion some of the commoner mistakes : "Acute gastritis" is a rare disease in adults. As a rule appendicitis or gallstones is the correct diagnosis. "Chronic indigestion" is usually a mistaken diagnosis, '"-e actual condition being peptic ulcer, pulmonary tuberculosis, constipation or cancer of the colon. "Bronchitis" usually proves to be phthisis, bron-°h iectasis or bronchopneumonia at autopsy or in the outcome. 'Asthma" beginning after middle life is usually tyuiptomatic of cardiac or renal disease. Unresolved pneumonia" is frequently a mistaken agnosis, the real disease being interlobar empyema. 'Malaria" is often given as the diagnosis in cases of Phthisis, hepatic syphilis, hepatic abscess and urinary "Sections. , typhoid fever" in a patient's history may moan Ubcrculosis or latent sepsis (septic endocarditis, 8uPPurative nephritis, etc.). Rheumatism" has sometimes turned out in my-'Pci'ience to mean: aortic aneurysm, cancer, of the Pl eura, tabes dorsalis, osteomyelitis, spondylitis deforf"8) bone-tuberculosis, syphilitic periostitis, lead-poisoning, morphin habit, alcoholic neuritis, trichiniasis and gonorrheal infection. "Rheumatism" is one of the most dangerous of all diagnoses to the conscientious
