Scientific inveStigAtionSStudy objectives: Obesity is an important risk factor for obstructive sleep apnea syndrome (OSAS), and weight loss can reduce apnea severity or even lead to resolution in some patients. Effective CPAP therapy may lead to weight loss by any of several proposed mechanisms, including, but not limited to, increased physical activity and increased responsiveness to leptin. This retrospective study sought to determine whether subjects who adhered to prescribed CPAP treatment for OSAS would lose weight, or gain less weight than control subjects who were either untreated or did not adhere to prescribed CPAP treatment. Methods: BMI was determined at the time of diagnosis and at followup approximately 1 year (10-14 months) later. Subjects who used CPAP ≥ 4 h per night and ≥ 70% of nights were considered treatment subjects. Control subjects used no treatment for OSAS or used CPAP < 4 hours per night or < 70% of nights for 1 year.Results: BMI of treatment and control subjects did not significantly differ (p = 0.3157). BMI increased with 1 year of CPAP use in women but not men (p = 0.0228) and in non-obese subjects (p = 0.0443). BMI did not significantly decrease in any group treated with CPAP. conclusions: CPAP was associated with weight gain in some; none lost weight. CPAP may affect weight in ways not measured here. Physicians should stress an active weight loss plan and not assume CPAP alone will lead to weight loss. A larger, prospective study may help clarify these findings.