This was a cross-sectional analytic study. In relation to pain from the sacroiliac joint, this study sought to establish 1) its prevalence, 2) the validity of pain provocation, 3) whether any arthrographic abnormalities predict a response to joint block, and 4) whether certain pain patterns discriminate patients with this diagnosis. The true prevalence of sacroiliac joint pain is unknown and despite a plethora of clinical tests, none of these tests has been validated against an established criterion standard. To our knowledge, arthrography of the sacroiliac joint had never been studied. Forty-three consecutive patients with chronic low back pain maximal below L5-S1 were investigated with sacroiliac joint blocks under image intensifier using radiographic contrast followed by 2% lignocaine, Information was obtained on pain provocation, analgesia, and image pattern. Thirteen patients (30%) obtained gratifying relief of their pain. Nine of these also exhibited tears of their ventral capsule. Groin pain was the only pain referral pattern found to be associated with response to sacroiliac joint block. The sacroiliac joint is a significant source of pain in patients with chronic low back pain and warrants further study.