OBJECTIVE : To validate the effectiveness of the selective wavelength filter analysis with infrared photographs for diagnosing small-angle esotropia in children under age 4. METHODS : This is a retrospective, observational, case-control study. A total of 83 esotropes with an esodeviation of ≤ 16 prism diopters (PD) and 75 orthotropic controls under 4 years of age were included. Full-face infrared photographs were taken with a selective wavelength filter in front of either eye. The angles of esodeviation on photographs were measured with the three-dimensional Strabismus Photo Analyzer. The alternate prism and cover test or the Krimsky test were repeatedly performed to measure ocular alignment. RESULTS : The testability of infrared photographs using selective wavelength filters in children under 4 years of age was 85.6%. The mean angle of esodeviation was 11.3 ± 4.0 PD by manual measurements and 11.5 ± 4.4 PD by the infrared photograph analysis. Manual measurements and the infrared photograph analysis showed a strong positive correlation (R = 0.815, P < 0.001). The sensitivity and specificity of the infrared photograph analysis for detecting small-angle esotropia were 95.2% and 77.9%, respectively, with a cutoff value of 4.0 PD. CONCLUSIONS : The automated infrared photograph analysis was simple and effective for diagnosing small-angle esotropia in young children.