Pseudoaneurysm of the left ventricle is an uncommon abnormality rarely diagnosed correctly before death. Usually in adults the underlying cause has been found to be severe coronary arterial disease (Hurst, Fine and Keyes, 1963; Bjornsson, 1964; Cone and Hawley, 1964; Roberts and Morrow, 1967, Chesleret al., 1969; Erseket al., 1969; Gobel, Vidsudh and Edwards, 1971) although in one previous case no abnormality was demonstrated by coronary ateriography (Gueronet al., 1973). Other causes include both penetrating (Jamshidi and Berry, 1965) and non-penetrating (O'Reilly, Kazenelson and Spellberg, 1970) chest trauma and rupture of a cardiotomy wound (Bross, 1967). The formation of a pseudoaneurysm carries a poor prognosis but surgical correction is feasible and may favourably alter the outcome, particularly if a correct and detailed anatomic diagnosis can be made prior to surgery (Gueronet al., 1973).This report describes the clinical and radiological features of a unique case of left ventricular pseudoaneurysm formation occurring in a patient without significant coronary arterial disease. The etiologic factor is thought to be rupture of a small preexisting left ventricular diverticulum occurring 36 hours after mitral valvar replacement.A 64-year-old woman presented in 1973 with a history of increasing exertional dyspnoea and orthopnoea. Mitral stenosis had been first diagnosed in 1955 and in 1966 increasing exertional dyspnoea was noted and cardiac catheterisation was performed.