The extent of heart disease and its relationship to the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), was studied with M-mode, 2D and Doppler echocardiography in 42 consecutive patients, 30 females and 12 males, median age 63 (range 23-75) years with histologically verified mid-gut tumour, liver metastases and 24-h urinary 5-HIAA excretion above 47 mumol.24 h-1. All patients had normal left ventricular ejection fractions, median 65% (interquartile range (IQR) 54-74%). Moderate to severe tricuspid regurgitation (TR) was diagnosed in 22 patients (59%); mitral or aortic regurgitation was found in nine (24%) and six (16%) patients, respectively. The mitral flow peak early (E) on late (A) velocity ratio was significantly decreased compared to age-matched normal subjects. The group of patients with 5-HIAA excretion exceeding 1000 mumol.24h-1 contained significantly more patients with severe TR than those with a lower excretion. The decrease in the E/A ratio may indicate reduced left ventricular compliance, possibly secondary to fibrous changes similar to those seen intra-abdominally and in the right side of the heart. As serotonin is degraded in the lung circulation, other mediators such as tachykinins and cytokines (PDGF) may be involved.