Apraxia of speech (AOS) is a rare disorder of motor speech programming, and few case reports have included sufficient description of both clinical findings and lesion localization. We report a case with an isolated progressive speech articulation deficit and brain involvement restricted to the left superior frontal gyrus. This case suggests that slowly progressive AOS may be a clinical disorder distinct from primary progressive aphasia, and that it can occur without language disorders or bucco-facial apraxia. Our findings highlight the importance of the left hemisphere in speech articulatory deficits and suggest that the left superior frontal gyrus plays a crucial role in specific articulatory processes.