To the editor: The point raised (4) that the low-estrogenic early follicular phase does not reflect women in general is valid as this phase only represents a fraction of the menstrual cycle and it is indeed important to understand how the different hormonal levels influence the vasculature. However, hormonal status should not be ignored when studying vascular function in women, in particular not when trying to isolate important physiological mechanisms by advanced invasive methodologies. In such studies, including hundreds of women with varying menstrual status is not practically feasible or ethically sound. Given the large redundancy that exists between important vascular control systems (2), the effect size of a given pharmacological blocker on, e.g., vascular conductance is typically relatively low(1, 3). Therefore, to evaluate a physiological mechanism, it is vital to control for factors which are known to significantly impact vascular conductance. Such factors may be external, i.e., caffeine intake or recent physical activity, or internal, such as the menstrual cycle. In controlling for hormonal fluctuations, we also recommend measurements of sex hormone levels at the day of the experiment, as hormonal levels can vary substantially between individuals when tested in the expected same phase. Research on vascular function in women is still in its infancy and due to the large hormonal fluctuations, conducting studies in premenopausal women is far more complex than studying vascular function in men, although differences in testosterone levels also are likely to be important. We encourage more advanced studies in women that consider hormonal status.