The alpha angle is a radiological measurement that has been proposed for the detection of cam morphology in patients suspected of femoroacetabular impingement syndrome (FAIS). After analysing published articles on FAIS, it is apparent that different types of circles are used in the measurement of alpha angles. To determine the interobserver and intraobserver reliability of alpha angle values measured using different types of circles on a 45° Dunn lateral radiograph in patients with symptomatic FAIS. The 45° Dunn lateral radiographs of the operated hips of patients who underwent hip preservation surgery in Dokuz Eylül University Hospital between 2014 and 2017 were evaluated retrospectively. Alpha angles were measured manually with transparent goniometers using three different circle types: thin full circle, thin-dashed circle, and thick full circle, and eye estimation without circle insertion. Measurements were made separately by an orthopaedist and a radiologist. A second measurement was made by the radiologist 1 month later for intraobserver reliability. Interobserver reliability for the thin full circle, thin-dashed circle, thick circle, and eye estimation (ICC = 0.645, 0.525, 0.494, and 0.588, respectively). Intraobserver reliability for the thin, thin-dashed, thick line, and eye estimation (ICC= 0.626, 0.681, 0.681, and 0.462 respectively). Interobserver difference of mean values of alpha angles for thin full, thin-dashed, thick full, and eye estimation were 2.7°, 0.9°, 1.1°, and 2.3°, respectively. Intraobserver differences between measurements were 1.5°, 0.9°, 0°, and 1.6°, respectively. Our study demonstrated that interobserver reliability is highest when measurements are made using a thin full circle.