This paper analyses disfluencies and ungrammatical expressions in the speech of 11-13-year-old Finnish-speaking boys with ASD (N = 5) and with neurotypical development (N = 6). The ASD data were from authentic group therapy sessions and neurotypical data from teacher-led group discussions. The proportion of disfluencies and ungrammatical expressions was greater in the speech of participants with ASD (26.4%) than in the control group (15.5%). Furthermore, a qualitative difference was noted: The ASD group produced long, complex disfluent turns with word searches, self-repairs, false starts, fillers, prolongations, inconsistent syntactic structures and grammatical errors, whereas in the control group, the disfluencies were mainly fillers and sound prolongations. The disfluencies and ungrammatical expressions occurring in the ASD participants' interactions also caused comprehension problems.