Rationale:Refeeding syndrome (RS) is a fatal condition caused by rapid calorie intake during starvation. Self-neglected fasting in psychiatric disorders is associated with RS. However, overeating resulting from circumventing the clinician’s instructions does not have a reportedly high risk of RS.Patient concerns:A 47-year-old undernourished woman with borderline personality disorder was hospitalized for nausea, vomiting, and diarrhea.Clinical findings:She had not eaten much for 10 days and had lost weight (56.5–51.1 kg) over 3 weeks. No abnormalities were indicated on physical examination and imaging examinations.Diagnoses, interventions, and outcomes:Infectious diseases and malignancies were excluded from the differential diagnosis. On the third day of admission, the patient’s serum phosphorus level significantly decreased to 0.7 mg/dL, and additional sodium phosphate was administered intravenously. On the fourth day, despite our instructions, the patient was found to be eating nonhospital food from the first day of admission. In conjunction with her history, a final diagnosis of RS was made. After appropriate treatments, the patient was discharged on the 15th day of hospitalization. The patient’s nausea, vomiting, and diarrhea were improved.Lessons:When undernourished patients have psychiatric disorders, including borderline personality disorder or schizophrenia, the occurrence of RS should be considered based on the patients’ poor adherence to physicians’ instructions.