BACKGROUND Molar incisor hypomineralization (MIH) is a developmental disturbance of enamel affecting the first permanent molars and permanent incisors. The epidemiological evidence on MIH prevalence shows it to be in the range of 3.6 to 25%. The prevalence of MIH has not been documented in Bulgaria which warranted the conduction of the present study. OBJECTIVE To find the prevalence of molar incisor hypomineralisation in Bulgarian children. METHODS The study included randomly selected 2960 children aged 7 to 14 years from Plovdiv and born between 1992 and 1999. The children were allocated to age groups which were matched in number--each of them included 370 children and consisted of approximately equal number of girls and boys. The defect was assessed by visual and tactile inspection using directed light without the enamel surface being preliminary dried. The prevalence of MIH was determined totally and for each age group; also we determined the degree of damage by type of teeth, and the average number of affected teeth of one person with MIH. The results were analysed using alternative analysis at a level of significance P < 0.05. RESULTS The mean prevalence of MIH of the studied populations was 3.58%. The prevalence of MIH for the different age groups was within the range of 2.43-7.84%. The highest prevalence was found for the children born in 1999 (7.84%). It was significantly higher in comparing all age groups (P < 0.05) with the exception of those born in 1992 (P > 0.05). The mean number of affected teeth per person with MIH was 3.99 of which 2.08 were first permanent molars, 1.86--incisors and 0.5--canines. The incisors and the first permanent molars were approximately equally affected (P > 0.05). CONCLUSIONS The results of the study suggest that molar incisor hypomineralization can be found in Bulgarian children too. The prevalence of MIH and the mean number of affected teeth of one individual are lower than the data reported in similar studies in other countries.