Purpose: Robust empirical evidence has illustrated a rising prevalence of body dissatisfaction and disordered eating among males, noting that both may be oriented more toward muscularitydas opposed to thinness-oriented concerns. While an elevated prevalence of premorbid anorexia nervosa (AN) has been noted among those with muscle dysmorphia, little evidence has examined the process of this transition from thinness-oriented to muscularity-oriented disordered eating. Methods: This clinical observation presents a case of AN in an adolescent male, whose symptom presentation transitioned during the course of treatment for AN, into more muscularity-oriented disordered eating. Results: Despite the core body image disturbance and disordered eating behaviors being retained, the behavioral expression of muscularity-oriented disordered eating was difficult to detect during treatment for AN, and he was discharged from treatment as his muscularity-oriented disordered eating escalated. Conclusions: Transdiagnostic crossover between thinness-oriented and muscularity-oriented disordered eating represents an important clinical concern, which may be challenging to measure and assess. Implications for treatment are discussed, and the early detection of muscularity-oriented disordered eating. (C) 2016 Society for Adolescent Health and Medicine. All rights reserved.