Background: This study investigated the effect of speech therapy in a heterogeneous group of adults with intellectual disability. Method: Thirty-six adults with mild and moderate intellectual disabilities (IQs 40-70; age 18-40 years) with reported poor speech intelligibility received tailored training in articulation and listening skills delivered in two 3-month periods. Pre- to post-changes in speech intelligibility and receptive vocabulary were assessed using standardized tasks. Results: The results showed a positive effect of treatment on speech intelligibility and receptive vocabulary, irrespective of severity of intellectual disability, hearing loss and intellectual disability aetiology. Conclusions: Speech therapy for people with intellectual disability can be effective at adult age and hearing loss should not prevent treatment. Continued attention to speech can help augment verbal communication skills in this population.