We studied the prevalence and the characteristic features of cyclical vomiting syndrome (CVS) in the defined childhood population of the City of Aberdeen, in Scotland. Initial screening was done by questionnaire in 10% of all children between 5 and 15 years of age (2,165 children) attending schools in Aberdeen, followed by clinical interviews of symptomatic children. We invited for clinical interview 69 children (3.9%) with a history of unexplained vomiting; 46 (67%) attended. Of them, 34 fulfilled the criteria for the diagnosis of CVS (prevalence rate, 1.9%). Children with CVS had a mean age of 9.6 years and a mean age at onset of symptoms of 5.3 years (range, 1-13). The overall sex ratio was 1:1, although in younger children boys were more commonly affected than girls. Seven children (21%) also suffered from migraine, 10 (29%) from travel sickness, and 10 (29%) from atopic diseases. The attacks of vomiting occurred on average eight times per year, with a mean duration of 20 h. Travel was a frequent precipitating factor. Attacks were commonly associated with pallor, anorexia, and malaise and were often relieved by rest and sleep. The clinical features of CVS overlapped to a large extent with those of migraine, suggesting a common pathogenesis. Features common to both conditions included trigger factors, associated GI, sensory, and vasomotor symptoms, and factors that relieved attacks. There was also an association between CVS and headache, abdominal pain, atopic diseases, and travel sickness.