PURPOSE: Rhombencephalosynapsis is a rare congenital cerebellar malformation increasingly recognized by prenatal and neonatal neuroimaging. Cognitive outcome seems to be variable but is not well documented. AIMS AND METHODS: To study neurological, behavioural, and cognitive functions of patients with non-syndromic rhombencephalosynapsis, five patients (three female and two male, mean age at the time of this study 8.9 years, range 4.3-17.3 years) were assessed by neurological examination and several tests of behaviour and cognitive functions. RESULTS: Ataxia was present in all patients, but daily life activities were partly restricted in only one. Other symptoms were muscular hypotonia, abnormal eye movements, and head stereotypies. Three patients had pathological scores on both attention and hyperactivity/impulsivity scales. Only two patients had normal full-scale IQ (IQ value of 109 and 114, respectively). Verbal and/or performance IQ were impaired in three. CONCLUSION: In non-syndromic rhombencephalosynapsis the clinical presentation is variable. Attention deficit and hyperactivity disorders are frequent behavioural problems. Cognitive functions are mostly impaired, as mild intellectual impairment without a typical cognitive profile. However, rhombencephalosynapsis is compatible with normal cognitive functions. No definitive correlation between cognitive impairment and additional supratentorial abnormalities could be established.