Sacroiliitis (say-kroe-il-e-I-tis) is a painful condition that affects one or both sacroiliac joints. These joints sit where the lower spine and pelvis meet. Sacroiliitis can cause pain and stiffness in the buttocks or lower back, and the pain might go down one or both legs. Standing or sitting for a long time or climbing stairs can make the pain worse. Sacroiliitis can be hard to diagnose. It can be mistaken for other causes of low back pain. It's been linked to a group of diseases that cause inflammatory arthritis of the spine. Treatment might involve physical therapy and medicines.