OBJECTIVES: To investigate distribution of affected teeth and severity of molar-incisor hypomineralisation (MIH) in 8-9-year-old children. A second aim was to study association between severity of MIH and hypersensitivity, caries, and affection of incisors and second primary molars (SPM). METHODS: A total of 3013 children in one age cohort participated in a cross-sectional study, of which 851 children were diagnosed with MIH. A majority of these children were re-examined and MIH diagnosis based on the European Academy of Paediatric Dentistry criteria was confirmed in 538 children. The re-examinations were undertaken at the local clinics by one calibrated dentist. Data were tested with bivariate logistic regression analysis. Results were reported using frequencies, proportions, odds ratios (OR) and confidence intervals (CI). RESULTS: Almost half of the children with MIH (46.8 %) had at least one severely affected molar. Incisors were affected in 51.9 % of children with MIH, and the prevalence was higher in children with severe affection (57.4 %, p < 0.01). Among children with MIH, second primary molars were affected in 29.6 %, hypersensitivity in at least one first permanent molar was reported by 25.8 and 30.8 % had caries extending to dentine. Children classified with severe MIH were more likely to suffer from hypersensitivity (OR 5.62, 95 % CI 3.61-8.74) and dentine caries (OR 10.32, 95 % CI 6.46-16.50) than children with mild MIH. CONCLUSION: Prevalence of hypomineralised incisors and SPM were high in the studied children with MIH. Children with severe MIH had higher probability of incisor affection, dentin caries and hypersensitivity. CLINICAL SIGNIFICANCE: This study highlights the importance of understanding the association between MIH, caries and hypersensitivity, especially in children with severe MIH. These children need extensive and individualized care in the dental services to prevent caries and pain.