: OBJECTIVE: The pineal hormone melatonin (MLT) has been proven to play a fundamental physiological regulatory role on both biological and psychic functions and alterations of the light/dark circadian rhythm of MLT have been described in several chronic immunoinflammatory diseases and in psychic disorders. Aim of the present biological explanatory study was the evaluation of MLT circadian rhythm in autistic children, in order to preliminary assess the pineal endocrine function in the autistic syndrome. METHODS: The study included 14 children suffering from classical infantile autism, who were investigated for the whole 24-hour circadian rhythm by collecting venous blood samples at 4-hour intervals. Serum levels of MLT were measured by the RIA method. The control group consisted of 20 age-matched healthy children. RESULTS: No autistic patient showed a normal MLT circadian rhythm. Moreover, autistic children showed significantly lower mean concentrations of MLT, mainly during the dark phase of the day, with respect to the values observed in the controls. CONCLUSION: The results of this preliminary study suggest the existence of a pineal endocrine hypofunction in autistic children, whose pathophysiological significance needs to be thoroughly investigated in successive clinical studies.