OBJECTIVE: To test the hypothesis that dieting, or the restriction of caloric intake, is ineffective because it increases chronic psychological stress and cortisol production-two factors that are known to cause weight gain; and to examine the respective roles of the two main behaviors that comprise dieting-monitoring one's caloric intake and restricting one's caloric intake-on psychological and biological stress indicators. METHODS: In a 2 (monitoring vs. not) × 2 (restricting vs. not) fully crossed, controlled experiment, 121 female participants were assigned randomly to one of four dietary interventions for 3 weeks. The monitoring + restricting condition tracked their caloric intake and restricted their caloric intake (1200 kcal/day); the monitoring only condition tracked their caloric intake but ate normally; the restricting only condition was provided 1200 kcal/day of food but did not track their calories, and the control group ate normally and did not track their intake. Before and after the interventions, participants completed measures of perceived stress and 2 days of diurnal saliva sampling to test for cortisol. RESULTS: Restricting calories increased the total output of cortisol, and monitoring calories increased perceived stress. CONCLUSIONS: Dieting may be deleterious to psychological well-being and biological functioning, and changes in clinical recommendations may be in order.