Some cases of thymus developmental defects (DiGeorge's syndrome) are associated with the development of defects of the heart and great vessels. To see if anomalies of the heart are also associated with anomalies of the thymus, a material of thymus tissue removed during operation for congenital heart disease was compared with thymus tissue from a forensic autopsy material representing the same age group, but without cardiac lesions. Among 27 cases of congenital heart disease, occurrence in the thymus of solid epithelial cords were seen in 6 and cyst- or duct-like structures were found in 7. Among 47 forensic cases, 1 showed cords and another showed duct-like structures. Of these, one had congenital anomalies in the form of hepatic hamartomas and nesidioblastosis, whereas the other had died from morbilli, raising the possibility of congenital immune deficiency. Anyhow, a striking occurrence of minor morphological deviations in the structure of the thymus was found in children with congenital heart disease. Such material therefore may be less suitable for studying the morphology and biology of the normal thymus gland.