Background: Investigating how fasting affects the hunger-fullness cycle, changes in body weight, body mass index (BMI), and percentage of fat from a weight regulatory perspective is of utmost importance. Objectives: The aim of this study was to compare the effects of fasting on appetite-regulating hormones in thin and obese females. Methods: In this quasi-experimental research, 25 thin and obese females, whose age ranged from 30 to 45 years were selected through the purposive convenient method. They were divided in terms of their BMI to 2 groups of thin (n = 13) and obese (n = 12) individuals. Their serum concentration of ghrelin and peptide YY (PYY) was then measured in both groups in 4 stages: 3 days prior to the fasting month, in 14 and 28 days of fasting, and 2 weeks after the fasting intervention. The collected data were then statistically analyzed via repeated-measure analysis of variance (ANOVA) (P < 0. 05). Results: The mean serum concentration of ghrelin in obese and thin females in the pre-intervention, intervention, and postintervention time series were, respectively, 109 to 119 and 142 to 192 ng/mL. Within-group differences were not statistically significant (P < 0. 05). The mean concentration of PYY in obese and thin females in the pre-intervention, intervention, and post-intervention time series showed change from 103 to 124 and 142 to 166 pg/mL, respectively. No statistically significant within-group differences were observed (P < 0. 05). Between-group differences of ghrelin concentration and PYY in the pre-intervention, intervention, and post-intervention time series were not statistically significant (P < 0. 05). Conclusions: The present findings revealed that fasting had no effect on the hunger-fullness cycle and that significant reduction in weight, BMI, and percentage of fat during fasting were independent of hormone changes.