Carcinoid syndrome may involve right carcinoid heart disease (CHD), secondary to the release of vasoactive substances. Left CHD is rare, as the inactivation of serotonin by the lung protects the left heart. We attempted to evaluate the prevalence of CHD and patent foramen ovale (PFO) with serial contrast transthoracic echocardiographic studies and to determine the markers of right and left CHD progression.Forty-one consecutive patients with proved digestive endocrine tumor and carcinoid syndrome were prospectively enrolled. All patients underwent serial conventional and contrast transthoracic echocardiographic studies. Right and left CHD, PFO, radiological examinations, and biological carcinoid markers were systematically assessed.Left CHD was present in 5 patients at baseline and in 13 patients (32%) during follow-up (P = .03). The 13 patients with left CHD also had PFO, and no left CHD occurred without PFO (P < .0001). Right CHD was present in 16 patients (39%) at baseline and in 25 patients (61%) at the end of follow-up (P = .04). The prevalence of right and left CHD was significantly higher in patients with PFO (88% and 76%, respectively; P < .04). A progression of right and left CHD was present, respectively, in 19 and 9 patients but was mainly found in patients with PFO (15 and 9 patients; P < .0001). The main marker of CHD progression was the presence of PFO (odds ratio 44.2, 95% confidence interval 4.4-447.7; P = .001).PFO is a new marker of CHD progression and should be systematically assessed with routine contrast transthoracic echocardiography in patients with carcinoid syndrome to determine patients at high risk of CHD progression.