The treatment of underweight and malnourished individuals with eating disorders (EDs) and obsessive-compulsive disorder (OCD) poses unique challenges and considerations. Low weight and malnourishment have implications on symptom presentation, treatment course, and outcomes. Clinicians should consider the impact of malnourishment on organ systems, the importance of medical care in treatment, and dietary considerations for weight restoration. Restrictive eating disorders such as AN and atypical AN can lead to similar psychological effects of malnourishment even if not underweight. Current ED treatment programs consider all patients as having malnutrition effects due to their illness, regardless of BMI. Atypical AN often goes unnoticed, leading to reinforcement of disordered eating behavior. Chronic malnutrition and starvation can affect almost every major organ system and result in permanent damage or death if not addressed. Additionally, underweight or malnourished patients often have a lot of dietary needs and need a lot of calories for restoration, which can be complicated by OCD symptoms such as obsessionality and intolerance of uncertainty.