Introduction: Cam impingement is characterized by abnormal contact between the proximal femur and acetabulum caused by a non-spherical femoral head, known as a cam deformity. A cam deformity is usually quantified by the alpha angle; greater alpha angles substantially increase the risk for osteoarthritis (OA). However, there is no consensus on which alpha angle threshold to use to define the presence of a cam deformity. Aim: To determine alpha angle thresholds that define the presence of a cam deformity and a pathological cam deformity based on development of OA. Methods: Data from both the prospective CHECK cohort of 1002 individuals (45-65 years) and the prospective population-based Chingford cohort of 1003 women (45-64 years) with respective follow-up times of 5 and 19 years were combined. The alpha angle was measured at baseline on anteroposterior radiographs, from which a threshold for the presence of a cam deformity was determined based on its distribution. Further, a pathological alpha angle threshold was determined based on the highest discriminative ability for development of end-stage OA at follow-up. Results: A definite bimodal distribution of the alpha angle was found in both cohorts with a normal distribution up to 60, indicating a clear distinction between normal and abnormal alpha angles. A pathological threshold of 78 resulted in the maximum area under the ROC curve. Conclusion: Epidemiological data of two large cohorts shows a bimodal distribution of the alpha angle. Alpha angle thresholds of 60 to define the presence of a cam deformity and 78 for a pathological cam deformity are proposed. (C) 2013 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.