Sacroiliitis (say-kroe-il-e-I-tis) is an inflammation of one or both of your sacroiliac joints — situated where your lower spine and pelvis connect. Sacroiliitis can cause pain in your buttocks or lower back, and can extend down one or both legs. Prolonged standing or stair climbing can worsen the pain.
Sacroiliitis can be difficult to diagnose, because 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 medications.