Purpose: Expressive language impairments are common among school-age boys with fragile X syndrome (FXS) and autistic boys. Given the high cooccurrence of autism spectrum disorder (ASD) among individuals with FXS, cross-condition comparisons can elucidate the specificity of such impairments as they relate to ASD. Language samples can provide fruitful information regarding individuals' grammatical skills in less structured formats relative to standardized measures. This study examined grammatical errors produced during a conversational language sample among 20 boys with FXS and cooccurring ASD (FXS + ASD) and 19 autistic boys matched on ASD severity. Method: Language samples were coded for omissions and errors at the word and utterance levels. Participants' grammatical errors were also compared to separate mental age--matched and mean length of utterance--matched boys from a reference database. Results: Boys with FXS + ASD and autistic boys produced similar rates of errors across all categories. Relative to their matched comparison groups, boys with FXS + ASD and autistic boys produced significantly more omissions during conversation. Conclusions: These findings suggest that omissions may be a unique grammatical marker associated with the ASD phenotype. Further examination of omissions across diagnostic groups would aid in clarifying the specificity of omissions in the language phenotype of ASD.