Background Rapid eye movement [REM] sleep behaviour disorder (RBD) may herald neurodegenerative diseases. Neurobiological deficits similar to those identified in neurodegenerative diseases have been reported in idiopathic RBD. Researchers are looking for early markers supporting a possible role of RBD as a harbinger of impending neurodegenerative disease. Objective To examine the neuropsychological functions in idiopathic RBD subjects. Should they be found to present a neuropsychological dysfunction that overlaps that reported in neurodegenerative diseases, it would be possible to consider cognitive deficits as possible early markers of an underlying degenerative process. Methods Twenty-three subjects with idiopathic RBD (21 males, mean age 67.0±7.0 years) and a group of healthy controls matched for sex, age and education underwent a neuropsychological battery evaluating different cognitive domains. Findings Considering mean values, poorer performances were observed in the Word Span (p <.001), Rey–Osterrieth’s complex figure recall (p =.003), Digit Span (p =.003) and Logic Memory (p =.003) tests. On the basis of equivalent scores, the RBD subjects performed significantly more poorly on tests of visuo-constructional learning abilities (p <.001). Interpretation Our data show the possible presence of cognitive deficits in RBD defined as idiopathic, sharing common features in particular with Lewy body disease. Neuropsychological evaluation in RBD could lead to presymptomatic identification of neurodegenerative disease, but until more prolonged long-term follow-up data are available, the true neurobiological significance of cognitive deficits in RBD will remain unknown.