Anesthesiologists may face difficulty maintaining the airway for general anesthesia or regional anesthesia in patients with ankylosing spondylitis (AS) to maintain their airway for general anesthesia or perform regional anesthesia due to ossification of the axial bone during surgery. However, low back pain is also a common problem in AS patients. Administration of caudal epidural injections to AS patients undergoing hip surgery has been reported. Nevertheless, few reports have addressed imaging techniques for chronic pain treatment in AS patients. We administered ultrasound-guided caudal epidural injections to three AS patients (all men, 48-64 years of age, with confirmed diagnosis from 5-20 years) to successfully reduce chronic pain (more than 50% pain relief). Under ultrasound guidance, we identified the ossification of the sacral hiatus and sacrococcygeal ligament and administered the injection without difficulty. Ultrasound-guided caudal block is a safe and effective approach for chronic low back pain in AS patients.