A simple and highly sensitive method for the identification and quantitative estimation of 5-hydroxyindoleacetic acid (5-HIAA) concentration in urine is described. 5-HIAA was concentrated following extraction into ethyl acetate, and separated from other indolic acids by paper chromatography. A color reagent, p-dimethylaminocinnamaldehyde, was used to detect 5-HIAA on paper, in concentrations as low as 0.1 µg./cm.2 Urinary excretion levels could be measured below the normal range as well as in the normal range and above.