Objective: The aim of this study is to estimate the incidence of endometriosis in a northwestern region of Italy. The potential sources of geographical variations in the incidence of endometriosis within the region are discussed. Methods: The patients selected were women between 18 and 45 years of age, born and residing in Piedmont who had undergone medical or surgical treatment for endometriosis between 2000 and 2005. The data were obtained from official hospital discharge records. Results: The number of women contributed to the study was 3,929. The age-standardized incidence rate of endometriosis was 81.8/100,000 patient-years (95% CI 79.1–84.2). The distribution of relative risks showed some areas with an increased rate of around 30% (southern and central Piedmont), while for other areas the disease risk was lower (southwestern Piedmont). These areas have greater exposure to environmental risk due to the presence of chemical pollutants. Conclusion: In order to achieve reliable data and good management of the disease, there is great need for national registers, as well as networks of excellence for the treatment of endometriosis. Our findings suggest that environmental factors may be associated with the development of the disease, but the observed results need to be cautiously interpreted in the context of ineligible biases.