Fifty hearts with a rupture of the free wall were studied. From a correlation between gross and microscopic features and the clinical history, several interesting features evolved. Firstly, the gross aspect of the rupture enabled a distinction of three types. Type I, characterized by an abrupt, slit-like tear, which correlated clinically with a recent infarct, mostly of less than 24 hours previously. Type II which showed an'erosion'of the infarcted myocardium, indicative of a slowly progressing tear. This type correlated with a somewhat larger time interval between onset of symptoms and tamponade. Type III, characterized by early aneurysm formation, which correlated clinically with older infarcts. Furthermore, occasionally, marked discrepancies were found between the histological and clinical dating. This lends support to the concept that in some patients the onset of symptoms may be secondary to the onset of rupture, in the setting of an otherwise silent infarction. Clinically, the acute episode is diagnosed as the onset of infarction. The term'concealed rupture'is proposed for this phenomenon.