PDD-NOS has been an illusive diagnostic category in Autism Spectrum Disorders (ASD). It is a frequently occurring ASD, but it has typically been defined by what it is not--autism. This latter condition has received the bulk of the attention in the development of diagnostic methods, while PDD-NOS has largely been ignored from a diagnostic standpoint. The symptoms that characterize PDD-NOS in adults with intellectual disability (ID), beyond the extrapolation of a few child studies, are largely unknown. This study is an attempt to provide systematic empirical data to describe the condition of PDD-NOS in adults with ID. The implication of these data for diagnosis and future research are discussed.