Levels of 5-hydroxy-3-indole acetic acid (5-OH IAA), a metabolite of serotonin, were studied in infants with infantile colic in order to investigate the aetiology of infantile colic pathogenesis. The study included 16 patients with infantile colic and 10 control subjects. Random urinary 5-OH IAA levels of colicky infants were found to be higher than those in the control group. This finding suggested that high serotonin levels may be responsible for infantile colic.