In the setting of a large hospital, and probably particularly in a teaching hospital, the dictum, "Do no harm," is often ignored in the rush to do all imaginable tests and "look for zebras" when at least the major diagnosis may be obvious. Three examples, encountered in a one-year period, of infants with primary cardiac or pulmonary diseases who suffered fatal or near fatal effects from an unnecessary lumbar puncture are the basis of this report. Case 1 A 4½-month-old black male presented in the emergency room with a one-day history of vomiting and diarrhea. The patient was a 1,710-gm first dizygotic twin born at 36 weeks to a 36-year-old multipara who had premature rupture of the membranes.