The objective of this study was to identify behavioral differences between children with multiple complex developmental disorder (MCDD) and those with pervasive developmental disorder-not otherwise specified (PDD-NOS). Twenty-five children (6–12 years) with MCDD and 86 children with PDD-NOS were compared with respect to psychiatric co-morbidity, psychotic thought problems and social contact problems using the child behavior checklist/4–18, the Dutch version of the diagnostic interview schedule for children—Version IV, the child and adolescent functional assessment scale, and the autism diagnostic observation schedule-generic. MCDD was associated with anxiety disorders, disruptive behavior, and psychotic thought problems. PDD-NOS was associated with deficits in social contact. MCDD differs from autistic disorder, and can also be delineated from PDD-NOS.