Foods high in refined sugar are claimed to exacerbate hyperactivity and increase aggressive behavior. Controlled studies have failed to confirm any effect on hyperactivity and effects on inattention have been equivocal. Possible effect on aggressive behavior has received little study. This study assessed cognitive attention and aggressive behavior immediately following an acute ingestion of sugar compared with saccharin and aspartame-sweetened placebos in 17 subjects with attention deficit disorder with hyperactivity compared with 9 age-matched control subjects. The sugar and placebo challenges were given with a breakfast high in carbohydrate. Although the children with attention deficit disorder with hyperactivity were significantly more aggressive than the control subjects, there were no significant effects of sugar or either placebo on the aggressive behavior of either group. However, inattention, as measured by a continuous performance task, increased only in the attention deficit disorder with hyperactivity group following sugar, but not saccharin or aspartame. This result is of questionable clinical significance inasmuch as aggressive behavior was unchanged. The finding may be due to the combination of the sugar challenge with a high-carbohydrate breakfast. These findings should be replicated and any possible clinical significance should be documented before any dietary recommendations can be made.