In patients with space-occupying hemispheric infarction, surgical decompression within 48 h after stroke onset increases the chance of a good functional outcome, but also the chance of survival with severe disability. Until now, cognitive outcome in these patients has not been reported in a consecutive series. Participants of the hemicraniectomy after middle cerebral artery infarction with life-threatening edema trial (HAMLET; ISRCTN94237756) underwent detailed neuropsychological examination at a median of 14.5 months after stroke onset. ‘Global cognitive impairment’ was defined as a score on the Cambridge cognitive examination (CAMCOG) ≤ 83. Impairment on an individual neuropsychological task was defined as a score below the 2.5th percentile or more than two standard deviations below standard norms. The association between cognitive and functional outcome was analysed with linear regression. Twenty patients were tested. Fifteen (75 %) had global cognitive impairment or such poor performance that assessment of global cognitive performance by the CAMCOG was not feasible. Five had only focal cognitive deficits. Still, detailed neuropsychological examination was feasible in 18 patients. Patients with aphasia performed worse than those without on both verbal and non-verbal tasks. Poorer cognitive performance was associated with worse functional outcome as assessed with the modified Rankin scale (β −0.4, 95 % CI −0.6 to −0.1). No differences were observed between operated and non-operated patients. The majority of survivors of space-occupying hemispheric infarction suffered from long-term global cognitive impairment. Isolated focal neuropsychological deficits were found in only a quarter. Impaired cognitive outcome was associated with worse functional outcome.