Context: Although it is well established that adiposity increases the severity of the clinical features of polycystic ovary syndrome (PCOS), the data regarding the prevalence of PCOS in obese women and the change in body weight women presented with PCOS over time are scarce. Objective: The objective of the study was to determine whether obesity increases the risk of PCOS and whether the degree of obesity of PCOS patients has increased, paralleling the rise in obesity in the population. Design: We analyzed data from two consecutive populational studies assessing the prevalence of PCOS and a database containing all untreated PCOS patients evaluated at a university clinic between 1987 and 2002. Setting: The study was conducted at a tertiary care center. Patients or Other Participants: Participants included 675 women who participated in prevalence studies and 746 PCOS patients. Main Outcome Measures: Populational prevalence of PCOS according to body mass index (BMI) and change in BMI of PCOS patients over time were measured. Results: The prevalence rates of PCOS in underweight, normal-weight, overweight, and obese women were 8.2, 9.8, 9.9, and 9.0%, respectively. Prevalence rates reached 12.4 and 11.5% in women with BMI 35-40 kg/m2 and greater than 40 kg/m2 (P = NS). The mean BMI of PCOS patients diagnosed between 1987 and 2002 rose, beginning in 1997 and reaching 37.3 ± 9.9 kg/m2 in 2000-2002, paralleling the change in BMI of the surrounding population (10-14% obesity rate in 1987, 15-19% in 1997, and 25% or greater in 2002). Conclusion: Our results suggest that the risk of PCOS is only minimally increased with obesity, although the degree of obesity of PCOS patients has increased, similar to that observed in the general population. These data indicate that obesity in PCOS reflects environmental factors to a great extent. Copyright © 2008 by The Endocrine Society.