Back pain is the most common symptom of ankylosing spondylitis (AS). The aggravation of back pain in AS patients is usually thought to be a flare-up of the disease activity. We report here on the case of a 31-year-old AS patient with low back pain. The lesions of this patient were unexpectedly diagnosed as metastatic spinal tumor due to cholangiocarcinoma. In a patient with AS, an acute aggravation of low back pain may be due to other pathologic conditions such as osteoporotic vertebral fracture, vertebral discitis and metastatic bone disease. Therefore, when a patient with AS presents with nonspecific low back pain, a physician should consider the above mentioned conditions.