In the population of live born children in the County of Vestfold, Norway, during the seven-year period 1982-88 (N = 15,307), 138 cases of congenital heart defects were diagnosed (patent ductus arteriosus in preterm infants excluded), an incidence of 0.9%. In 114 infants (83%) the defect was diagnosed before discharge from hospital after birth (nursery, neonatal unit), in 20 infants (14%) it was diagnosed later during the first year of life, and in four (3%) during the second year of life. In 24 children (17%) congenital heart defect was associated with a syndrome (Down syndrome eight, Edwards syndrome three, other syndromes three), or other congenital malformations (single eight, multiple two). Diagnosis was made clinically only (including ECG, phonocardiography and X-ray pictures) in 15 patients (11%). 13 were classified as having ventricular septal defects, and two were unclassified. Echocardiography was performed in 120 children (87%), heart catheterization in 44 (32%), surgery in 47 (34%), and autopsy in 12 (9%). 15 children (11%) died, all during the first year after birth. 45 children (33%) are healthy after spontaneous closure of a ventricular (41 children) or atrial septal defect (four children), and 15 (11%) after surgical repair. 63 (46%) are alive with a defect. We found a tendency towards increasing incidence of congenital heart defects. This increase was explained by echocardiographic diagnosis of small muscular ventricular septal defects in the early neonatal period, with spontaneous closure of the defect during the first year of life.(ABSTRACT TRUNCATED AT 250 WORDS)