A follow-up study was performed on 256 patients with ankylosing spondylitis. HLA-B27-negative Dutch patients very rarely had classical idiopathic ankylosing spondylitis. Most of these patients had psoriasis or inflammatory bowel disease or they were foreigners living in The Netherlands. Many of the B27-negative patients only have sacroiliitis. Psoriasis or inflammatory bowel disease among the HLA-B27-positive with a classical ankylosing spondylitis is rare.