Deficiency of tryptophan with elevated serum serotonin and liver dysfunction are the prerequisites for the experimental production of cardiac lesions in the guinea pig model of carcinoid syndrome. To apply the above principles in human subjects with carcinoid disease, various indole markers were compared in patients with or without heart involvement, to a group of normal subjects. In the present study, plasma tryptophan (T), serotonin (5HT), and urinary 5 hydroxyindoleacetic acid (5HIAA) measurements were made in 18 (group 1) patients with carcinoid syndrome and 24 normal individuals (group 2). Of the 18 patients, seven (group 1A) had valvular involvement and 11 (group 1B) had none, as determined by clinical, roentgenographic, and echocardiographic (M-mode and 2-D) techniques. Analysis of the above data shows that unlike animal models, there is no difference in serum tryptophan, serum serotonin, and urinary 5 hydroxyindoleacetic acid levels, in patients with carcinoid syndrome with or without cardiac involvement (p > 0.05). Furthermore, the data confirm that serum tryptophan, a substrate in carcinoid syndrome, is decreased and in serum serotonin and urinary 5 hydroxyindoleacetic acid, the metabolites are elevated.