OBJECTIVE: Low-carbohydrate (L-CHO) diets are often used for weight loss but their effects on cognitive function are not well understood. The present study compared the effects of a L-CHO and high-carbohydrate (H-CHO) weight-loss diet on cognitive function adults. DESIGN: Participants were randomized to either a L-CHO (n=22) or H-CHO (n=25) weight-loss diet. Cognitive function was evaluated by four computerized cognitive tasks (Stroop Task, Continuous Performance Task, Word Recall and Wisconsin Card Sorting Task) presented in random order before and at 1, 4, 12 and 24 weeks after the initiation of the L-CHO or H-CHO diet. PARTICIPANTS: Forty-seven adults (25 males) with a mean±s.d. age of 47.4±8.7 years and body mass index of 35.3±3.4 kgm-2. RESULTS: There were no significant differences in weight loss between groups at any time point. There were significant improvements on color Stroop task accuracy over time in both diet groups (P<0.05), but there were no differences in performance between groups on this or any other cognitive task at any time period. CONCLUSION: These findings suggest that weight loss has neither a positive nor a negative effect on cognitive function and that L-CHO and H-CHO weight-loss diets have similar effects on cognitive performance. © 2013 Macmillan Publishers Limited All rights reserved.