PURPOSE: Maintenance of cognitive abilities is important for elderly to stay independent. With the aging of the population, the call for modifiable factors is emerging. Dietary protein might improve cognitive performance; however, this has hardly been studied. Therefore, we studied the impact of 24-week dietary protein supplementation on cognitive performance in pre-frail and frail elderly people. METHODS: Pre-frail and frail elderly subjects, according to the Fried criteria, randomly received a protein drink containing 15 g protein or a placebo drink twice a day. Cognitive performance was measured at baseline and after 24 weeks by means of a sensitive neuropsychological test battery. In addition, reaction time was assessed after both 12 and 24 weeks of intervention. Domain scores were calculated for the domains episodic memory, attention and working memory, information processing speed, and executive functioning. Analyses of covariance were used to determine differences between groups. Linear mixed models were used to determine differences in reaction time over time and per treatment. RESULTS: In total, 65 subjects (79 +/- 8 years) with a median Mini-Mental State Examination score of 28 (interquartile range 26-30) were included. Reaction time improved more in the protein group (68 ms) than in the placebo group (18 ms, P = 0.03). Dietary protein had no significant effect on any of the cognitive domain scores. CONCLUSIONS: Protein supplementation might improve reaction time performance in pre-frail and frail elderly, but did not improve other cognitive functions.