OBJECTIVE To find the prevalence of molar-incisor hypomineralization (MIH) in a random sample of Spanish children, and to investigate the gender influence, distribution of defects, the treatment need associated and the relation between this disorder and dental caries. METHODS A cross-sectional study was carried out to determine MIH and caries prevalence in a randomly selected sample of 840 children from the 8-year-old population of the Valencia region of Spain. The examinations were carried out in the children's schools by one examiner who had previously been calibrated with the MIH diagnostic criteria of the European Academy of Paediatric Dentistry (EAPD). RESULTS The percentage of children with MIH was 21.8% (95% CI 19.1-24.7), with a mean 3.5 teeth affected (2.4 molars and 1.1 incisors) been the maxillary molars the most affected. No gender differences were found. Of those with MIH, 56.8% presented lesions in both molars and incisors Children with MIH needed significantly more urgent and non-urgent treatment than those without MIH (chi-squared test P-value < 0.005). Both caries indices were significantly higher (Student's t-test P-value < 0.05) in the children with MIH than in the healthy children: the DMFT scores were 0.513 and 0.237 and the DMFS scores 1.20 and 0.79, respectively. CONCLUSIONS Molar-incisor hypomineralization prevalence is high in the child population of this region and equally affects boys and girls. The condition increases significantly the need of treatment of affected children. A significant association with dental caries was observed.