Seventy-three consecutive patients with a left superior vena cava evaluated at the Mayo Clinic, Rochester, Minnesota, between 1983 and 1987 underwent cardiac catheterization and two-dimensional echocardiography. Bilateral superior venae cavae were present in 89%. Entry of the left superior venae cavae was into the coronary sinus in 62% (4% were unroofed), a pulmonary venous atrium in 21%, and a common atrium in 17%. Catheterization successfully identified the left superior vena cava in all patients; two-dimensional echocardiography was successful in 68% (group 1) and unsuccessful in 32% (group 2). There was no significant difference between groups with regard to age, sex, diagnosis, or site of drainage. In group 1, 43% had a dilated coronary sinus; in group 2, the coronary sinus was present in 61% but was of normal size. Cineangiograms revealed smaller caliber left superior venae cavae in group 2 than in group 1 (means 7.4 and 11.3 mm, respectively). Thus two-dimensional echocardiography is not totally reliable for the detection of small but possibly significant left superior venae cavae.