In twenty-five patients with a clinical diagnosis of suspected sacroiliitis conventional radiography, CT and MRI were performed. In ten patients no abnormalities were demonstrated. In thirteen cases CT and MRI revealed sacroiliitis. In two patients with normal plain films and CT para- and intraarticular changes of signal intensity suggested suspicious sacroiliitis. MRI can be considered as an important imaging modality for early diagnosis of sacroiliitis. In eighteen patients with a firm diagnosis of ankylosing spondylitis and plain films of the thoracolumbar junction suggesting destructive Romanus and Anderson inflammatory lesions MRI was done. Two distinct groups of inflammatory changes were found. In ten patients MRI findings compatible with active inflammatory enthesitis were revealed at the disco-vertebral junction. In eight cases focal and linear changes of signal intensity within the intervertebral disks suggested an active inflammation. Using MRI the spectrum of inflammatory changes in sero-negative spondylitis can be presented. In sixteen patients with definite clinical diagnosis (psoriatic arthritis--thirteen cases and Reiter's syndrome--three cases) plain films and MRI of small hand joints were performed. The patients fell into two distinct groups. In the first MRI findings could not be differentiated from those seen in rheumatoid arthritis. In nine cases the distribution and extent of soft tissue findings were different, similar to changes seen in enthesitis. Therefore, on the basis of MRI findings in small peripheral joints easier differential diagnosis between sero-negative spondyloarthritides and rheumatoid arthritis is possible. In five patients with a diagnosis of Reiter's syndrome having clinical signs of enthesitis plain films and MRI of calcaneus were done. MRI revealed findings compatible with active inflammation which resembled those seen at the attachment of the annulus fibrosus and collateral ligaments of the small hand joints.