The purpose of this study was to investigate the relationship between serum complement C3 levels and BMI in adolescent and adult females participating in nutrition therapy for anorexia nervosa and eating disorder not otherwise specified. Specific objectives of this study were to investigate the following: (1) whether complement C3 levels differ in adult patients with a BMI level < 18.5 kg/m? as compared to those with a BMI level ≥ 18.5 kg/m²; (2) whether complement C3 levels differ in adolescent patients with a BMI between the 15^th-14^th percentile versus a BMI between the 15^th-85^th percentiles; and, (3) whether a correlation exists between complement C3 levels and % IBW in adult and adolescent patients. A convenience sample of 38 subjects participated in this study. Results showed that adults with BMI levels < 18.5 kg/m ² had significantly lower complement C3 levels compared to those with BMI levels ≥ 18.5 kg/m², There was no significant difference in complement C3 levels in adolescents between BMI percentiles and there was no significant correlation between serum complement C3 levels and % IBW in adults and adolescents combine