Speech, language, and cognitive impairment are defining features of Kleefstra syndrome but have not been systematically characterised, limiting robust clinical decision making and future planning. We characterised speech, language, and cognition in 103 individuals with Kleefstra syndrome. Language and cognitive ability ranged from average to severely impaired and most verbal individuals had speech production disorders (48/49, 98%). Developmental regression occurred (11/80, 14%). Those with speech disorders or who were minimally verbal used communication aids such as sign language or speech generating devices (61/103, 59%). The heterogeneous communication and cognitive profile necessitates tailored speech intervention and early access to communication aids. [By Lottie Morison MSpPath, BA, BSc, GCertMHSc, CPSP (she/her), https://jmg.bmj.com/content/early/2024/01/30/jmg-2023-109702 ]