A patent foramen ovale (PFO) was looked for in 500 consecutive adult autopsy studies (265 men, 235 women) in which death was due to acquired cardiovascular pathology (mainly coronary artery disease); this condition was demonstrated in 73 cases (42 men, 31 women); 14.6%. A PFO forms a short inter-atrial communication (average length 5 mm) directed anteriorly and slightly superiorly, opening in the right atrium anteriorly at the junction of the fosse ovale-limbus and in the left atrium under membranous fold concave anteriorly, which represents the anterior expansion of the valve of the fosse ovale. The average surface area of the PFO is 0.5 cm2 (range 0.2 to 1.5 cm2). The underlying cardiac pathologies, age, sex, had no relation to the frequency of PFO. Of the different analysable anatomical features, weight of the heart, atrial size, tricuspid regurgitation, texture of the fosse ovale (thickness over 1mm, or thin, transparent membrane), dimensions of the Eustachian valve, had no influence on the frequency of PFO. No thromboses were observed incarcerated in, in contact with or adherent to the right atrial surface of the PFO. Three of the 73 patients with PFO had, shortly before death, experienced an embolic cerebrovascular accident: in all 3 cases there was a thrombus in the left heart chambers.