Recent evidence has suggested that persistence of the left superior caval vein is associated with a high incidence of obstructive lesions of the left heart. To shed more light on this issue 1085 patients with congenital heart disease were studied retrospectively, with the aim of estimating the prevalence of a persistent left superior caval vein and its associated anomalies, focusing attention on obstructive lesions in the left and right ventricles. Patients with isomerism of the atrial appendages, or hypoplastic left heart syndrome, were excluded. A persisting left superior caval vein was present in 57 patients (5.2%). The overall incidence of obstructive lesions of the left heart was higher in patients with than in those without a persistent left superior caval vein (31.6 versus 7.8%,p < 0.001). Relative hypoplasia of the left ventricle was also higher in patients with persistent left superior caval vein (14 versus 0.8%, p < 0.001). The obstructive lesions found in the left heart, compared with the number in those without a left caval vein, were: mitral stenosis, 5.2 versus 0.7%; subaortic stenosis, 5.3 versus 0.9%; aortic coarctation, 17.5 versus 5.8% (p < 0.01); all of these in association, 3.5 versus 0.4%. In contrast, the incidence of obstructive lesions of the right heart was similar in the two groups of patients. It is concluded that persistence of the left superior caval vein can perturb the normal development of the left ventricle, being strongly associated with obstructions to left ventricular inflow and outflow.