Carcinoma of the breast: measurement and the management of treatment. I. The value of the data. This is the first of a series of papers in which we shall explore some insights into the biological changes which accompany the treatment of human tumours which may be obtained through estimation of volume changes in relation to treatment. We have adopted a working hypothesis that regression slopes reflect the composition of individual tumours and, indirectly, their intrinsic growth rate rather than the effectiveness of treatment. The breast has proved to be a suitable site for measurement and our interpretation of the results has led to the development of a new style of management for carcinoma of the breast: measurement based sequential therapy (MBST). In this paper the method of measurement and detailed statistical evaluation of the quality of the data from 262 patients (263 tumours) is presented. Exponential regression lines have been fitted to describe volume changes in relation to treatment by radiation, chemotherapy and hormones. A simple classification of steepness of slopes is introduced. 