There is increasing evidence supporting the effectiveness of the Early Start Denver Model (ESDM) for children on the autism spectrum. However, substantial variability in response to the ESDM has been reported across participants. We examined the plausible yet untested hypothesis that variations in the fidelity level of therapists delivering the intervention contribute to variability in children's response to the ESDM. Videotaped sessions (n = 40) of toddlers on the autism spectrum who received the ESDM from trained therapists were coded to obtain measures of therapist fidelity and children's learning in response to the therapists' instruction. Variations in overall fidelity, along with variations in most items included in the ESDM fidelity checklist, contributed to the children's learning response during the sessions.