Sternal bone marrow puncture was performed on 300 patients in the Mount Sinai Hospital. This procedure is virtually diagnostic in acute leukemia, "aleukemic" leukemia, multiple myelomata, leish-maniasis, malaria, and Gaucher''s disease. It is confirmatory in chronic leukemia in all its phases. In pernicious anemia it may help establish the nature of the anemia to evaluate therapy. It has also proved of value in estimating the reactive capacity of the bone marrow. It is of inestimable value in a negative sense in many conditions mimicking blood dyscrasias where the elimination of a specific blood dyscrasia is a matter of great import (e.g. infectious raono-nucleosis).