This study is an attempt to define the variables responsible for the differences in the mortality rates after radical mastectomy for mammary cancer among women treated in the Barnes Hospital between 1912 and 1933 (Series A) and women treated more recently in the Ellis Fischel Cancer Hospital (1940-1950) and in the Barnes Hospital (1950-1955) (Series B). The question particularly kindling this investigation was whether or not improved therapeutic techniques had contributed significantly to the lower mortality rates from mammary cancer in the more recent years. Materials and Methods Two population samples were investigated: (1) 351 mammary cancer cases seen in the Barnes Hospital between 1912 and 1933 and (2) 789 mammary cancer cases treated in the Ellis Fischel Cancer Hospital (1940-1950) and in the Barnes Hospital (1950-1955). The latter Series (B) has been described.5 The former group (Series A) included 102 cases diagnosed without histological examination. The tissue sections