Following a 1-week baseline, 19 normal boys (mean age 9.8 +1.8 years) ingested either caffeine (5 mg/kg) or placebo twice a day for a 2-week period per condition in a double-blind crossover study (total study duration of 5 weeks) to study the behavioral, autonomic and side effects of caffeine. Mothers of the whole sample and children who were low caffeine users could distinguish between drug conditions by side effects. Caffeine increased autonomic reactivity of low users only. Behavioral and autonomic results were ambiguous for high users indicating possible caffeine withdrawal symptoms. While 'caffeinism' may occur in children, either self-selection and/or tolerance may prevent its occurrence in naturally selected diets.