This literature review, from 1990 to the present, discusses the characteristics of autism and the comorbidity of mental retardation and autism. Specific medical syndromes that complement the heterogeneity concept are described, including epilepsy, fragile X syndrome, Rett syndrome, tuberous sclerosis, and Asperger syndrome. The paper presents some genetic and medical factors in the diagnosis of autism and delineates research prospects and priorities. Results of the review indicate that autism remains a complex syndrome that seems to be more clearly defined in parameters of neurobiological and neurobehavioral and psychological descriptors. The research substantiates the organic and genetic indices of autism, but finds that the differential diagnosis is still not simple and presentation of symptoms may overlap with other atypical syndromes. Co-morbidity with mental retardation and autism is found to occur in as many as 75 percent of the cases. The construct of a continuum, compendium, or spectrum of disorders is discussed, with consideration for subgroups to explain the types of autism presented in research and clinical findings. The contributions of research in the fields of neurobiology and neuropsychology, and the technical advancement in genetic research and neuroimaging are addressed. (Contains 73 references.) (CR)