Summary311 hearts of fetuses, of premature and neonate babies, of preschool and school children were examined, irrespective of basic disease and cause of death. 63 hearts did not show any pathological alterations. They served as a control group. The fibers of 27 hearts of fetuses, premature and mature neonate babies were homogeneous" The close packing as well as the reduced number of myofibrils in some regions of the heart did not correspond to hearts of the same age and size of the control group.The following changes were found individually or in combination within the remaining 200 hearts:Perinuclear vacuoles and acidophilic fibers, acidophilic necroses, scars with or without calcification, interstitial haemorrhages, parietal thrombi, endocarditis and myocarditis.Prominent characteristics are perinuclear vacuoles, acidophilic fibers and acidophilic necroses. This combination often shows a characteristic morphological pattern: inner layer with acidophilic necroses, middle layer with acidophilia, and outer layer consisting of fibers with perinuclear vacuoles. This combination and the trilaminar arrangement lead to the conclusion that they may be caused by hypoxaemia, isehemia or hypoxidosis. Vacuoles and acidophilia are most likely to be reversible.According to their frequency, they appear to correlate with certain diseases (table 3), but none of the changes are specific for a certain disease:Perinuclear vacuoles, acidophilic fibers and acidophilic necroses very often exist with pulmonary diseases, with general infections and erythroblastosis. Interstitial haemorrhages and parietal thrombi can be traced in increasing numbers in traumas showing disseminated intravascular coagulation.Hypertrophic fibers are frequent in congenital heart disease, mostly in combination with acidophilic fibers and especially with scars.Further studies should aim at establishing a correlation between morphological and clinical observations. This paper suggests further studies in the field of morphology examining equivalent surgical material with the electron microscope.