This study was designed to examine the classification performance of diagnostic rules for pervasive developmental disorder not otherwise specified (PDD-NOS) and multiple complex developmental disorder (McDD), with clinical diagnosis as the gold standard. McDD is an heuristic concept of a developmental disorder characterised by social impairments, affective dysregulation, and thought disturbance. Detailed information on the symptoms, reliably extracted from the charts of 103 children with PDD-NOS and McDD, 32 with autistic disorder, and 96 with non-PDD disorders, was used to determine the presence of the DSM-IV criteria of autistic disorder and the criteria of McDD. A scoring rule for PDD-NOS based on a short set of seven DSM-IV criteria with a cut-off point of three items and one social interaction item set as mandatory had the best balance between high sensitivity and high specificity. The most effective and simple rule based on McDD criteria had a cut-off of three items, out of six items of anxieties and thought disturbance.