It will be a pilot, 2 year, prospective, randomized, double-blind, placebo-controlled (for pamidronate) study. All patients with AS will receive treatment with tumor necrosis factor (TNF) inhibitor, as indicated by local guidelines. Randomization will be performed for pamidronate vs placebo group. Pamidronate will be prescribed as monthly intravenous infusion in the dose of 60 mg/month for the first 6 consecutive months of every study year. Primary outcome will be the rate of radiographic progression of ankylosing spondylitis (AS), calculated after 24 months of combined treatment. Clinical and laboratory disease parameters will serve as secondary outcomes. These outcomes as well as safety assessments will be performed on a monthly basis up to 24 months.