This paper describes the existence of severe developmental speech dyspraxia in a member of a family that is typical of other reports of X-linked mental retardation without physical abnormality (Renpenning syndrome). Other family members have evidence of motor or speech dyspraxia. Other reports of X-linked mental retardation have mentioned "verbal disability", which suggests that developmental dyspraxia may be quite common in these families. Developmental dyspraxia of speech is amenable to specific types of intervention: after two years of such intervention our patient's adaptive skills reflect his normal non-verbal intellectual ability, despite persistent speech deficits. This functional improvement shows how important it is to identify dyspraxia in young children, who may otherwise be labelled mentally retarded.