What other disorders that occur as separate disorders in conjunction with autism (comorbid conditions) must be taken into account in the diagnosis and assessment of autism, Asperger syndrome, Rett syndrome, and childhood disintegrative disorder? Most, but not all persons with A also have some degree of mental retardation. There is now a very clear expectation in behavioral research that degree of mental retardation and severity of language deficit must be considered in designing and interpreting studies of the autism spectrum disorders. These co-occurring factors (mental retardation and degree of language deficit) have received less consideration in biomedical research. Lack of control for mental retardation and language deficits may be one reason why the replication rate of findings (i.e., multiple investigators obtaining the same findings) within biomedical research has typically been much lower than within behavioral research. The importance of diagnosis of comorbid (co-occurring) conditions such as affective disorder (e.g., depression) or obsessive-compulsive disorder, particularly in adults with autism spectrum disorders is also recognized, but standard procedures to do so are not well established. These await further research. It is particularly important to clarify the difference between true comorbidity and other ways in which symptoms from autism and different disorders may overlap.