Preclinical studies have demonstrated that conditioned fear extinction is impaired in females with low endogenous levels of the sex hormone estradiol, due to menstrual fluctuations or hormonal contraceptive use. As fear extinction is a laboratory model of exposure therapy for anxiety and trauma disorders, here we assessed the hypothesis that treatment outcomes may be diminished when exposure therapy occurs during periods of low estradiol. 90 women with spider phobia (60 cycling and 30 using hormonal contraceptives) underwent a one session exposure treatment for spider phobia, following which, serum estradiol levels were assessed. A median split in estradiol level was used to divide cycling participants into two groups; lower and higher estradiol. Behavioral avoidance and self-reported fear of spiders were measured pre-treatment, post-treatment, and at a 12 week follow-up assessment. Women using hormonal contraceptives exhibited a significantly slower rate of improvement across treatment, greater behavioral avoidance at post-treatment and follow-up, and fewer self initiated post-treatment exposure tasks, relative to both groups of cycling women, who did not differ. No group differences in self-reported fear were evident. Correlational analyses revealed that across the whole sample, lower estradiol levels were associated with slower rates of improvement across treatment, and greater self reported fear and behavioral avoidance at post-treatment, but not follow-up. These results provide the first evidence of an association between endogenous estradiol, hormonal contraceptive use, and exposure therapy outcomes in spider phobic women. Hormonal profile may partly account for variability in responsiveness to psychological treatments for anxiety and trauma disorders in women.