Although it remains a mysterious disorder since its description over a century ago, cyclic vomiting syndrome appears to be more prevalent than previously thought. With the goal of improving detection of affected children, two quantitative historical criteria-peak intensity > or = 4 emeses/h) and a episode frequency < or = 9/ month-can differentiate those with an explosive, intermittent cyclic pattern from those with a low-grade, daily chronic pattern. Diagnoses that can cause a cyclic vomiting pattern includes abdominal migraine, chronic sinusitis, intracranial neoplasm, anomalies of and mucosal injury to the gastrointestinal tract, urologic abnormalities, and metabolic and endocrine disorders. Because many organic disorders can mimic cyclic vomiting syndrome, a systematic diagnostic evaluation of potential underlying disorders is recommended. The following terminology is proposed: a cyclic pattern is differentiated from a chronic pattern of vomiting on the basis of a higher peak intensity of emesis and lower frequency of episodes; and of those with a cyclic vomiting pattern, an idiopathic subgroup who is not found to have an underlying disorder on diagnostic testing can be labeled as having cyclic vomiting syndrome.