Language samples from matched groups of 80 autistic, mentally handicapped, and normally developing children were coded for the presence of neologisms and/or idiosyncratic language use. Cognitive, social, or linguistic factors that might account for these errors were identified and assessed. More autistic subjects used neologisms and idiosyncratic language than age- and language skill-matched control groups. No single factor or combination of factors was responsible for this difference. Across diagnostic groups, similar patterns of error were noted, except that the autistic subjects were more likely to use words inappropriately that had no phonological or semantic similarity to the intended English word. For the autistic groups, the frequency of idiosyncratic language increased with language complexity. In contrast, such errors decreased with language skill in the mentally handicapped group. Theoretical implications for determining the nature and source of the language disorder associated with autism are discussed.