Background: Undiagnosed and early ankylosing spondylitis (AS), especially in adolescent patients suffering from back pain, may present with the finding of vertebral osteitis on MRI. Aims: To identify the early MRI changes of vertebral osteitis in AS. Patients and methods: Five patients (three boys, two girls) aged 11-20 years (mean 15.4 years) suffering from back pain underwent MRI of the thoracolumbar spine. There was no initial diagnosis of AS. After clinical and radiological suspicion of AS., MRI of the sacroiliac (SI) joints was performed. Results: During the course of AS, destructive and reactive changes affect the discovertebral junctions that are initially seen in the thoracolumbar area. At this stage plain radiography of the spinal column may be normal. On MR images, inflammatory osteitis of the vertebrae is seen as hypointense areas on T1-weighted images and hyperintense areas on T2-W images. The lesions enhance homogenously with contrast material. Conclusions: Awareness of the MRI appearances of vertebral osteitis is helpful in suspecting AS. Radiological examination of the SI facilitates the diagnosis and unnecessary further imaging can be avoided.