The aim of this study was to investigate psychosocial, cognitive, and motor functioning in patients clinically suspected of Sotos syndrome and to examine differences between patients with deletions or mutations of the gene encoding nuclear SET domain-containing protein 1 (NSD1; the major cause of the syndrome) and those without such alterations. Twenty-nine participants (21 males, 8 females) clinically suspected of Sotos syndrome (mean age 11y 10mo [SD 10y 11mo], range 1y 10mo–48y 5mo) were divided into an NSD1 mutation group (n=12; 8 males, 4 females) and an NSD1 non-mutation group (n=17; 13 males, 4 females). Intelligence, behaviour problems, attention-deficit–hyperactivity disorder (ADHD) symptoms, temperament, adaptive behaviour, and motor functioning were assessed with an extensive test battery. Scores were compared with those of control groups, and scores of the two subgroups were compared with each other. The mean IQ in the 21 individuals tested was 76 (SD 16; range 47–105). High rates of behaviour problems were found and patients lagged 1y 7mo to 2y 7mo behind in aspects of adaptive behaviour. In comparison with a control group of patients with a learning disability, motor functioning was better. NSD1 mutation compared with NSD1 non-mutation patients showed easier temperament, and fewer NSD1 mutation patients scored in the clinical range for ‘total behaviour problems’ (3/11 vs 13/17), ‘internalizing behaviour’ (2/11 vs 11/17), and ADHD (0/9 vs 4/15).