Research criteria proposed by Apley for identification of children with recurrent abdominal pain (RAP) have been widely adopted, but many researchers have deviated from the original definition. Ambiguities in Apley's criteria and problems caused by departures from his definition are discussed in this article. Many inconsistencies in research results may have been created by the decision of some researchers to exclude children with presumed organic causes for their pain from RAP study samples. Further precision and comparability of research results may be gained by using the two-stage approach to classification that is proposed in this article. The first stage requires no medical evaluation but is based strictly on the correspondence of RAP symptoms to temporal and severity criteria. At the second stage, subgroups of RAP are specified by using results from medical assessments or detailed patterns of symptoms. It is also suggested that researchers specify the temporal features of RAP and provide measures of impairment. Like Apley's criteria, the revised approach to classification allows the comparison of community- and school-based samples with children who have undergone medical evaluation for RAP.