Causal association between progesterone concentrations and breast cancer has not been shown

EDITOR—Increasing progesterone concentrations are not associated with increasing incidence of breast cancer, despite the neat correlation diagram by Jasienska and Thune, and they are wrong to infer causality.1 Some points in their diagram are based on 20 or fewer samples, from which they draw conclusions about the whole country, and many of the progesterone values quoted do not coincide with their cited reference.2 However, the main criticism is that they have ignored other scientific data that do not reinforce their hypothesis.

In some places high progesterone concentrations have been found in conjunction with a low incidence of breast cancer—for example, Shanghai, where the progesterone concentrations of small numbers of Chinese women were similar or even higher than women in Chicago or Harvard.3^–5 This is despite the fact that the incidence of breast cancer is 19.1 per 100 000 …