To examine how a low-carbohydrate diet affects cognitive performance, women participated in one of two weight-loss diet regimens. Participants self-selected a low-carbohydrate (n=9) or a reduced-calorie balanced diet similar to that recommended by the American Dietetic Association (ADA diet) (n=10). Seventy-two hours before beginning their diets and then 48 h, 1, 2, and 3 weeks after starting, participants completed a battery of cognitive tasks assessing visuospatial memory, vigilance attention, memory span, a food-related paired-associates a food Stroop, and the Profile of Moods Scale (POMS) to assess subjective mood. Results showed that during complete withdrawal of dietary carbohydrate, low-carbohydrate dieters performed worse on memory-based tasks than ADA dieters. These impairments were ameliorated after reintroduction of carbohydrates. Low-carbohydrate dieters reported less confusion (POMS) and responded faster during an attention vigilance task (CPT) than ADA dieters. Hunger ratings did not differ between the two diet conditions. The present data show memory impairments during low-carbohydrate diets at a point when available glycogen stores would be at their lowest. A commonly held explanation based on preoccupation with food would not account for these findings. The results also suggest better vigilance attention and reduced self-reported confusion while on the low-carbohydrate diet, although not tied to a specific time point during the diet. Taken together the results suggest that weight-loss diet regimens differentially impact cognitive behavior.