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2 | P a g e Several studies suggest that gender may affect cystic fibrosis (CF) disease severity, with women with CF being more severely affected. In this context it has been suggested that sex hormones may influence the CF phenotype. A large proportion of women with CF regularly use oral contraceptives (OC), but the effect of their use on disease severity is unclear. Here, we retrospectively assessed the effects of OC on clinical outcomes in women with CF. Data from 681 women were available of whom 42 % had taken OC for varying periods of time. We first performed an inter-patient analysis comparing annual change in %predicted FEV1, body mass index (BMI) and total days of intravenous (IV) antibiotic use over a five year study period in 57 women exposed to and 57 women not exposed to OC. There were no differences between the two groups. We next performed an intra-patient analysis of the same outcomes over a three year period of OC exposure and a three year period of no OC exposure in the same patient (n=23-27), but again did not detect any differences in any of the clinical outcomes. Our data suggests that the use of OC does not affect CF disease severity