PURPOSE: Myopia, a leading cause of correctable visual impairment, is projected to affect nearly 50% of the global population by 2050, posing a significant public health challenge. Understanding its prevalence and associated factors, particularly in children, is crucial for devising prevention and intervention strategies. This study aims to determine the proportion of myopia in school-aged children in Portugal and to examine the correlation between myopia occurrence and various environmental and genetic factors. METHODS: A cross-sectional epidemiological study was conducted on children aged from 5 to 17 years from nine schools in Lisbon, Portugal, between September 2020 and May 2021. It included optometric assessments to evaluate refractive status and binocular vision, as well as questionnaires about their lifestyles and parental myopia. RESULTS: Out of 1992 participants enrolled, 12.7% of the children were found to be myopic. The proportion of myopia increased with age and was higher in girls. A significant association was observed between myopia and parental history, with the likelihood being higher if one or both parents were myopic. Engaging in outdoor activities was associated with a lower likelihood of myopia. CONCLUSIONS: The study found that 12.7% of the children in the study sample, aged 5-17 years, were myopic, indicating a significant association with familial history and limited outdoor activities. These insights highlight the need for targeted myopia screening and prevention strategies in the paediatric population.