Objectives: One of the few successful treatments for obesity is bariatric surgery. About half of bariatric surgery patients are women of reproductive age. Gastric bypass, a common bariatric surgical procedure, is associated with malabsorption of nutrients and medications. Since OC are still the most popular method of reversible contraception, and are often used by obese women for noncontraceptive benefits, there is a need for better studies to evaluate OC use after gastric bypass. Objectives of the study were to evaluate the pharmacokinetics (PK) of oral contraceptive hormones in women before and after bypass surgery, using women as their own controls; and to assess the pharmacodynamics effects on ovarian follicle development, endometrial thickness, and cervical mucus. Methods: We planned a prospective study to collect data in this area. We planned to enroll 16 women planning gastric bypass surgery, with no contraindications to OC use. Women would take one 21-day cycle of OC in the months before their surgery, and one cycle 4 months after surgery. We will collect pharmacokinetic (PK) data for serum OC hormone levels, and correlate results with physiologic evidence of ovulation and contraceptive effect. We will compare standard PK parameters between groups. We hypothesized that malabsorption after gastric bypass will affect serum OC hormone levels, and subsequently, contraceptive efficacy, and that these effects will not be offset by any weight loss that may occur in the first few post-surgical months. We planned to compare this prospectively collected data to historical data from a group of non-obese women in a study with similar methods. Results: Due to inability to recruit (low number of eligible patients and ultimately no one who wanted to commit to participation), no results to report.