This study investigated the occurrence, nature, and severity of speech, language, and cognitive impairment in 76 children (61 males, 15 females) with isolated sagittal synostosis (ISS) aged 9 months to 15 years 7 months. There was no increased prevalence of global cognitive impairment in the group but there was a high prevalence rate of speech and/or language impairment with 28 (37%) displaying impairment of whom 20 (71%) had moderate or severe impairments that fulfilled the criteria for specific impairments. Prevalence rates were only increased for children over two years of age. Expressive language impairment occurred most frequently. Raised intracranial pressure, peri-neonatal risk factors, otitis media, or being operated were not associated with impairment. Surgery at a later age and a family history of speech and language impairment were both associated with impairments but numbers were small. The findings suggest that children with ISS are at an increased risk of developing speech and language impairment.