<p>Colour vision deficiency (CVD) is the inability or reduced ability to clearly differentiate different colours under normal lighting conditions. The study was conducted to determine the prevalence of red-green colour vision defects among schoolchildren at Repi Primary School in Addis Ababa, Ethiopia. School-based cross-sectional survey was employed involving 2400 healthy subjects (females = 850 and males = 1550), and the participants were from ten self-identified ethnic groups. The Ishihara pseudo-isochromatic 24 plate edition was used to test the colour vision of students under natural daylight conditions. The prevalence of congenital colour vision deficiency was 2.3%; 3.2% of males and 0.6% of females were affected by red-green colour vision defects. The prevalence of unclassified, deutan, and protan in male subjects were 0.20%, 2.5%, and 0.80%, respectively. Red-green colour vision defects were detected in subjects from 8 ethnic origins – Amhara(17), Dorze(8), Guraghe(5), Kembata(1), Oromo(15), Silte(7), Tegaru(2), and Wolaita(1). Both deutan and protan were highest among schoolchildren of Amhara ethnic origin: 2.9% and 1.1%, respectively. Our results underline the importance of early screening of CVD, particularly for schoolchildren. Proper screening, education and counseling are needed to minimize impacts of red-green colour vision defects, and can also be beneficial to the affected children in tackling difficulties in everyday work and for proper choice of future profession. </p>