The diagnosis of a left superior vena cava (LSVC) is important in planning the repair of a congenital heart defect as it may be the source of a sizeable right-to-left shunt. The medical record of 65 consecutive pts with a LSVC evaluated at the Mayo Clinic between 1980-1986 were reviewed. All pts had cardiac catheterization (CATH) and two-dimensional echocardiography (2D-ECHO) performed. The M:F ratio was 5:4; the mean age was 8.3 years. Bilateral SVC were present in 59 (91%) and a unilateral SVC was present in 6 (9%), 3 with absent RSVC syndrome. Eighty-one percent of the LSVC entered the coronary sinus CS (11% entered an unroofed CS), 8% entered a pulmonary venous atrium, and 11% entered a common atrium. CATH was used successfully to identify a LSVC in 100% of pts. 2D-ECHO, utilizing contrast and color flow in some pts, was used successfully in 86% (grpl; p=0.008), and unsuccessfully in 14% (grp2). There was no significant difference between the two groups with regard to age, gender, diagnosis, type of SVC or place of drainage. In grpl, a dilated CS was identified in 61%, contrast was used to identify a LSVC in 23%, and 25% were identified as a venous structure entering an atrium. When present the CS was of normal size in grp2 pts. Of the 9 pts, 7 had contrast used and 3 had color flow performed. We conclude: 1) Using 2D-ECHO, a LSVC was successfully identified in only 86% of our pts. 2) The failure to identify 14% could not be attributed to patient age, gender, type of congenital heart disease, type of LSVC or place of drainage. 3) If a suspected LSVC is not identified by 2D-ECHO, other non-invasive means, such as MRI, should be employed.