Resume Introduction and Objectives In the last decades, a congenital defect of the enamel's mineralization known as Molar Incisor Hypomineralisation has become a worldwide growing concern for health professionals.The aim of this paper lies on the investigation of the prevalence and possible etiology related to the appearance of Molar Incisor Hypomineralisation in children aged between 7 and 9 years old. These children are patients in clinic of the Oporto University's Faculty of Dental Medicine. Materials and Methods The sampling was constituted by 70 children born between 2004 and 2006 showing, at least, one partially erupted permanent molar, who were seen in a Pediatric dentistry appointment at the Clinic of the Oporto University's Faculty of Dental Medicine. Results The prevalence of Molar Incisor Hypomineralisation in children who participated in this study was 13% with a greater incidence on the female sex (67%) and in the jawbones (13%). The majority of diagnosed MIH damages were moderate (89%). Regarding the different episodes related to children aged up to 4 years old, bronchiolitis, urinary infection, pneumonia and the frequent use of antibiotics (amoxicilin) were the problems most associated with hypomineralisation diagnosis. Clinical Implications Epidemiological studies suggest that the prevalence of MIH is increasing. It is important that professionals are able to establish a difference between different enamel defects, clinical signs must be carefully evaluated for the treatment success. Conclusions More studies should be conducted with more representative sample to draw more conclusions. Keywords: Molar Incisor hipomeralization