Sacroiliitis (say-kroe-il-e-I-tis) is an inflammation of one or both of your sacroiliac joints — the places where your lower spine and pelvis connect. Sacroiliitis can cause pain in your buttocks or lower back, and may even extend down one or both legs. The pain associated with sacroiliitis is often aggravated by prolonged standing or by stair climbing.
Sacroiliitis can be difficult to diagnose, because it may 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 of sacroiliitis may involve a combination of rest, physical therapy and medications.