OBJECTIVE: To investigate the effects of weight loss diets on mood, food cravings, and other self-reported symptoms. RESEARCH METHODS AND PROCEDURES: Mood and other symptoms were evaluated by participant self-report using the Atkins Health Indicator Test (AHIT) in individuals undergoing weight loss following either a low-carbohydrate, ketogenic diet (LCKD) or a low-fat diet (LFD). Participants were 119 overweight community volunteers randomized to an LCKD or an LFD. An additional 51 participants who had completed an earlier trial contributed data for the psychometric analyses but were not included in the prospective analyses. Self-reported symptom levels on seven scales factor-analytically derived from the AHIT (negative affect, fatigue, somatic symptoms, physical effects of hunger, insomnia, hunger, and stomach problems) were acquired during 12 visits. RESULTS: After adjusting for the change in BMI over the course of the trial, participants experienced significant improvements in most symptoms regardless of diet. Diet group x visit interactions were observed for negative affect [F(9,803) = 2.30, p = 0.015] and hunger [F(9,803) = 3.62, p < 0.0002]. Examination of means indicated that the LCKD group reported less negative affect and hunger, compared with the LFD group. DISCUSSION: Regardless of diet, participants experienced significant improvement in a broad range of symptoms. Symptoms of negative affect and hunger improved to a greater degree in patients following an LCKD compared with those following an LFD. Whether these symptom changes explain the greater short-term weight loss generally experienced by LCKD followers deserves further research.