Evidence is presented that the growth rate of breast carcinomas in a defined population was lower in January and June than in the later months of the year. The tumours presenting later were on average larger and of higher histological grade. While there was no difference in the distribution of oestrogen receptor (ER) status, the receptor positive showed higher values later in the year. These high values were accompanied by a lack of tubular differentiation, again in keeping with a more rapid growth rate. Grading factor 1 is thus associated with the level of receptor positivity, in contrast to receptor status which is linked to factor 2. This indicates that ER- and ER+ tumours are derived from two distinct populations, and that a continuous scale is not involved.