The diagnosis of spondyloarthropathy is based on radiography of the sacroiliac joints, beside the patient's history and clinical examination. In cases where the clinical examination and radiography yield discrepant findings, contrast-enhanced magnetic resonance imaging (MRI) is a sensitive modality for the diagnosis of early sacroiliitis. Knowledge of the morphologic anatomy of the sacroiliac joints and of their abnormal micro- and macroanatomy in sacroiliitis and enthesitis are helpful for interpreting MR images. Arthritis of the sacroiliac joints is characterized by subchondral sclerosis, erosions, transarticular bone bridges, accumulation of periarticular fat, juxta-articular osteitis, synovtis, capsulitis, and enthesitis. The major histologic finding in active sacroiliitis is the presence of proliferative, pannus-like connective tissue destroying cartilage and bone. This tissue contains fibroblasts and fibrocytes as well as T cells and macrophages with a shift of the CD4/CD8 ratio toward the CD4 T helper cell population. The well-established grading of MRI findings by means of a chronicity and activity index, which are determined quantitatively from dynamic MR images, is supplemented by an alternative, semi-quantitative grading of activity. The following grades were defined for the short tau inversion recovery (STIR) sequence or the T1-weighted, fatsaturated spin-echo sequence for each quadrant (iliac anterior, iliac posterior, sacral anterior, sacral posterior): 0: no signal increase, 1: local increase in the joint cavity or within erosions, 2: small areas of increased juxta-articular signal, 3: moderate sized areas of increased juxta-articular signal, 4: large areas of increased juxta-articular signal. Values of the 4 quadrants are summed to an activity score (range 0-16). The new grading system is proposed to facilitate the examination and shorten image interpretation time.