BACKGROUND Superior vena cava syndrome (SVCS) is a clinical expression of obstruction of blood flow through the superior vena cava. The patterns of the Doppler flow changes of superior vena cava (SVC), especially the respiratory effects on them have not yet been fully elucidated. This study was to examine SVC Doppler flow patterns and the respiratory effects on them in healthy subjects and patients with SVCS. METHODS The SVC Doppler flow patterns of 18 normal human subjects and 22 patients with SVCS were analyzed at initial diagnosis and were followed up every 2 months for at least 11 months. RESULTS Among the 22 patients, 5 patients with the tumor near the right atrium oppressing the inferior segment of the SVC had clear VR- and AR-waves, while in the other 17 patients the VR- and AR-waves disappeared or their outlines were vague. The respiratory variations of the S- and D-waves as a percentage change in inspiration compared to expiration in patient group were much lower than those in control group (S-wave: 1.67 +/- 3.32% vs. 15.65 +/- 16.15%, P = 0.0003; D-wave: 1.80 +/- 1.12% vs. 23.55 +/- 37%, P = 0.0087), which gradually became larger with treatment and showed no significant difference with those in control group after 7 months. CONCLUSIONS The Doppler flows of the patients with SVCS correlate well with the images of CT scan of them. The respiratory variation of the S- and D-velocities could be used to evaluate the severity of SVC obstruction and its therapeutic effect.