The purpose of this work was to study the concentrations of the sex steroids estradiol and progesterone as well as those of cortisol, and the relationships between them in asthmatic and normal women. Blood samples were taken on the 5th day and 21st day of the menstrual cycle. It was found that at least one of the hormones was out of normal range in 80% of asthmatic women. It was possible to differentiate three groups of patients according to the plasma concentrations of progesterone on the 21st day of the cycle: (1) normal concentration of progesterone (56.7%): (2) concentration below the normal limits but still with the characteristic increase of the 21st day (16.8%): and (3) very low concentrations of progesterone, similar to those concentrations of the follicular phase (26.5%). The fact that there is a high number of asthmatic patients with at least one hormone out of the normal range of concentration whereas in the control group only one was found suggests that the occurrence of bronchial asthma is associated in a high proportion with abnormalities in the production or metabolism of steroid hormones in women during their reproductive life.