Background: Approximately one-quarter of individuals with fragile X syndrome (FXS) meet diagnostic criteria for autism; however, it is unclear whether individuals with comorbid FXS and autism are simply more severely affected than their peers with only FXS or whether they have qualitatively different profiles of behavioural impairments. To address this issue, variation in the FXS linguistic phenotype was examined in males with FXS with and without autism. The syndrome-specificity of the expressive language impairment of both groups of those with FXS was assessed in relation to Down syndrome. The extent to which different language sampling contexts affected expressive language in each diagnostic group was also examined. Method: Spontaneous language samples were collected from male adolescents with FXS without autism (n = 20), comorbid FXS and autism (n = 8), and Down syndrome (n = 16). Syntactic complexity (indexed by mean length of utterance), expressive vocabulary (indexed by lexical diversity), talkativeness, fluency and intelligibility were assessed in two contexts: conversation and narration. Groups were matched on non-verbal IQ, non-verbal mental age and chronological age to allow the assessment of relative strengths and weaknesses across language variables. Results: Males with comorbid FXS and autism were less intelligible than males with only FXS; no other differences between these two groups were found. Participants' performance differed across contexts for syntactic complexity, lexical diversity, talkativeness and fluency. Conclusions: These findings contribute to existing research on the behavioural profiles of individuals with FXS or FXS with autism who have low cognitive abilities. Although individuals with comorbid FXS and autism may be, as a group, more impaired than those with only FXS, data from this small sample of males with comorbid FXS and autism with low IQs suggest that their relative strengths and weaknesses in spontaneous expressive language are largely comparable and not differentially affected by the context in which their talk occurs.