Daily urinary excretion of 5-hydroxy-3-indoleacetic acid (5-HIAA) and 5-methoxyindole-3-acetic acid (5-MIAA) was studied by chromatography-mass-spectrometry in patients with cancer and healthy subjects. A considerable increase in urinary 5-MIAA-excretion was shown in patients with cancer of the stomach, rectum and lung. In some cases, a correlation was found between high urine levels of carcinogenic 5-MIAA and similarly high levels of 5-HIAA excretion with urine.