Background: Low-carbohydrate diets are often used to promote weight loss, but their effects on psychological function are largely unknown. Objective: We compared the effects of a low-carbohydrate, high-fat (LCHF) diet with a conventional high-carbohydrate, low-fat (HCLF) diet on mood and cognitive function. Design: Ninety-three overweight or obese participants [x Ȁ SEM age: 50.2 Ȁ 0.8 y; body mass index (in kg/m 2 ): 33.6 Ȁ 0.4] were randomly assigned to an energy-restricted (Ȃ6 -7 MJ, 30% deficit), planned isocaloric LCHF diet or an HCLF diet for 8 wk. Body weight and psychological well-being were measured by using the Profile of Mood States, Beck Depression Inventory, and Spielberger State Anxiety Inventory instruments at baseline and fortnightly. Cognitive functioning (working memory and speed of processing) was assessed at baseline and week 8. Results:The LCHF diet resulted in significantly greater weight loss than did the HCLF diet (7.8 Ȁ 0.4 and 6.4 Ȁ 0.4 kg, respectively; P ҃ 0.04). Both groups showed improvements in psychological well-being (P 0.01 for time), with the greatest effect occurring during the first 2 wk, but there was no significant difference between groups. There were no significant between-group differences in working memory (P ҃ 0.68), but there was a significant time ҂ diet interaction for speed of processing (P ҃ 0.04), so that this measure improved less in the LCHF than in the HCLF diet group. Conclusions: Both dietary patterns significantly reduced body weight and were associated with improvements in mood. There was some evidence for a smaller improvement in cognitive functioning with the LCHF diet with respect to speed of processing, but further studies are required to determine the replicability of this finding.Am J Clin Nutr 2007;86:580 -7.