Carcinoid heart disease is a morphologically specific type of cardiac disorder that involves the mural and valvular endocardium on the right side of the heart. Twenty-one subjects (57 percent) (Group I) with carcinoid heart disease and 15 subjects (43 percent) (Group II) without carcinoid heart disease were studied at necropsy. The two groups were similar in mean age (54 years versus 55 years), duration of clinical illness (4.7 years versus 6.3 years), body weight (50 kg versus 52 kg), systemic blood pressure (117/77 mm Hg versus 128/77 mm Hg), blood hematocrit levels (37 percent versus 36 percent), total serum protein levels (6.0 g/dl), and serum albumin levels (2.2 g/dl versus 2.6 g/dl). The two groups were different in the frequency of the presence of precordial murmurs consistent with tricuspid regurgitation and/or pulmonic stenosis (95 percent versus 13 percent), cardiomegaly by chest radiography (38 percent versus 0), low voltage on electrocardiography (47 percent versus 0), and location of the primary site of the carcinoid tumor. Total electrocardiographic 12-lead QRS voltage was similar in each group (105 mm versus 132 mm) (10 mm = 1 mV). Of Group I subjects, 43 percent died of cardiac causes; none of the Group II subjects died of cardiac causes. Of the 21 subjects with carcinoid heart disease, seven had left-sided cardiac involvement, but in none was it of functional significance. Thus, although carcinoid heart disease frequently is the cause of death in patients with the carcinoid syndrome, the development of carcinoid heart disease is not related to the duration of symptoms of the carcinoid syndrome.