Simple and reliable diagnostic aids need to be available for clinicians to consider sacroiliac joint dysfunction in the differential diagnosis of low back pain. The Fortin finger test was used as a means to identify patients with low back pain and sacroiliac joint dysfunction. Provocation-positive sacroiliac joint injections were used to ratify or refute the applicability of this new clinical sign for identification of patients with sacroiliac joint dysfunction. Sixteen subjects were chosen from 54 consecutive patients by using the Fortin finger test. All 16 patients subsequently had provocation-positive joint injections validating sacroiliac joint abnormalities. A subset of 10 individuals underwent additional evaluation to exclude the possibility of confounding discogenic or posterior joint pain sources. All 10 patients had no indication of either discogenic or zygapophyseal joint pain generators. These results indicate that positive findings of the Fortin finger test, a simple diagnostic measure, successfully identifies patients with sacroiliac joint dysfunction.