Achalasia is a disorder of the esophagus characterized by decreased lower esophageal sphincter (LES) relaxation and absence of esophageal peristalsis. Botulinum toxin (BT) injection targeting the LES has been used in the treatment of achalasia and other esophageal motility disorders, and it is considered to be safe and effective for short-term symptomatic relief. Serious adverse events of this procedure are rare. We report the case of an 83-year-old woman treated with BT injection for achalasia who subsequently developed a subdiaphragmatic abscess.