Severe hypomineralized first permanent molars (cheese molars) can be found in children. The aetiology of this phenomenon is unknown. The aim of this study is to collect more information about the causes of such molars. Parents of 24 children with severe cheese molars and of 21 controls without cheese molars, matched for age, living area and sex were interviewed. The mean age of the 45 children was 9.9 year (sd: 2.02). A questionnaire about the medical data from birth to four years of age and the medical situation of the mother during pregnancy was sent to the parents. No significant differences were found concerning weight and length at the time of birth between the two groups. Also no significant differences were found for the period of breast-/and bottle-feeding and for problems of the mother during pregnancy and child delivery. The mothers were healthy during pregnancy. Compared to the control group the children with cheese molars were ill more often. Significant differences were found for pneumonia, high fever and inflammation of the middle ear.