Cohen syndrome is an autosomal recessive, multiple congenital anomalies/mental retardation (MCA/MR) syndrome, caused by a mutation in the COH1 gen, localized on chromosome 8q22. COH1 encodes a transmembrane protein of 4.022 amino-acids with a presumed role in vesicle-mediated sorting and intracellular protein transport. Clinical features are non progressive psychomotor retardation and microcephaly, characteristic facial features, retinal dystrophy, and intermittent neutropenia. Examination of the long-term evolution of 6 patients with Cohen syndrome shows that the clinical features are rather stable during evolution. Description of their actual behavior on the basis of standardized questionnaires shows that no severe behavior problems are observed in any of the 6 patients. Taking into account their mental age, their behavior is quiet and easy to handle by their environment.