From a survey of 24-hour caffeine intake of 798 grade-school children (mean age, 10.3 years), 19 "high consumers" (reported intake of 500 mg/day or more) and a matched group of 19 "low consumers" were recruited for a double-blind, placebo-controlled, caffeine challenge study. Children received 5 mg/kg of caffeine twice a day or placebo for two weeks each, using a crossover design. While not receiving caffeine, high consumers had higher scores on an anxiety questionnaire and tended to have lower autonomic arousal (less frequent spontaneous skin conductance response and lower skin conductance level). While receiving caffeine, low consumers were perceived by their parents as more emotional, inattentive, and restless, while high consumers were not rated as changed. These differences cannot be attributed to tolerance, withdrawal, or subject selection, and suggest a possible physiological basis in children for dietary caffeine preference.