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Cohen syndrome is a rare autosomal recessive syndrome with a distinctive clinical phenotype that includes mental retardation and a characteristic sociable disposition. Variability in the level of learning disability and the behavioural phenotype is seen in the published literature. In a cohort of Finnish Cohen syndrome patients, severe mental retardation and non-maladaptive behaviour were described. Outside of Finland, autistic-spectrum behaviour has been reported in a few isolated Cohen syndrome patients but in a recent UK study was found to be highly prevalent. We report the results of neuropsychological studies in a group of 16 genetically heterogeneous patients, all with the characteristic clinical features of Cohen syndrome. Of the 9 patients who underwent formal neuropsychological testing, all but one was functioning in the severely mentally impaired range. Of the remaining patients, 3 were below the age of formal testing and 4 had such profound learning and behavioural problems that they were deemed unable to participate in testing. Mild maladaptive behaviour was observed in 13 patients and 3 were documented as having significant maladaptive behaviour. In contrast to the Finnish group of Cohen syndrome patients, this UK study identifies significant neuropsychological impairment combined with maladaptive behaviour as a characteristic of Cohen syndrome. Although autistic-type behaviour was observed, an increased prevalence of autism in Cohen syndrome was not confirmed.