The aim of this study was to determine possible etiological factors for a developmental enamel defect, i.e. demarcated opacities, affecting the permanent first molars. A questionnaire about possible etiological factors of enamel developmental defects was filled in by the parents of 8-yr-old children (n=516) prior to a dental examination of permanent teeth. Demarcated opacities of permanent first molars had been found in 18.4% of the children in a previous study of these children. Fifteen % had more than one tooth affected indicating systemic causation. Questions were asked about mother's health and medication during pregnancy, birth complications, health and medication of the child during the first 3 yr of life, breast-feeding, heredity, and fluoride supplements. The affected children, especially the boys, were reported to have had more health problems, in particular asthma (but only 4 cases), during the first year of life. Use of antibiotics was also more common among the affected children, but owing to a strong co-variance with health problems these factors could not be separated. Breast-feeding history was similar in children with and without enamel defects. The etiology of hypomineralized first molars is not yet fully understood, but based on the results of this retrospective study, health problems in infancy, especially respiratory diseases, seem to be involved.