The efficacy and tolerability of slow-release diclofenac sodium were investigated in 40 patients with ankylosing spondylitis. The patients received one coated tablet of 100 mg daily in the morning for 21 days in addition to standard physiotherapy. The efficacy of the trial treatment was evaluated by measuring the chin-to-manubrium sterni and fingertips-to-floor distances, Schober's sign, and chest expansion. Every morning and evening the severity of pain and the degree of spinal stiffness were recorded by the patients on visual analogue scales. A significant improvement of the symptoms was found in 35 of the patients as compared to the initial findings. The slow-release formulation of diclofenac Na was very well tolerated. The necessity of balneophysical therapy in ankylosing spondylitis is emphasized, which in most cases will only be possible in combination with adequate drug therapy.