BACKGROUND: Molar incisor hypomineralisation (MIH) is a developmental defect of decreased enamel mineral density, involving at least one first permanent molar and frequently, permanent incisors. The defects are demarcated and opaque, varying from white, yellow, and/or brown lesions to having post-eruptive enamel breakdown, with various clinical consequences. Mexican population studies have reported prevalence between 6.8 and 37.7%. AIM: To determine the prevalence, distribution and associated factors of MIH in Mexican schoolchildren aged 6-13 years from Pachuca, Mexico. MATERIALS AND METHODS: A cross-sectional study including 714 participants, selected randomly from nine public schools in Pachuca, Mexico. A dentist trained and standardised in the MIH-criteria of the European Academy of Paediatric Dentistry performed the clinical oral examinations, and questionnaires were developed to gather information regarding prenatal and perinatal health of the mother and child, as well as family sociodemographic and socioeconomic data. RESULTS: The prevalence of MIH was 12.3% (95%CI 9.5-14.5%). The average age of schoolchildren with MIH was 9.1 ± 1.7 years, and without MIH was 8.7 ± 1.8 years; the mean number of affected first permanent molars was 2.4 ± 1.1 and 1.1 ± 1.4 for incisors. The factors statistically associated, age (OR = 2.46), fathers' education (OR = 0.53), familial structure (OR = 4.69) and presence or absence of siblings (OR = 0.38). CONCLUSIONS: White demarcated opacities were the most prevalent type of defect, the severity of the lesions increased with the number of affected teeth. The factors associated with socioeconomic position were related to an increase in prevalence and severity.