Sacroiliitis and sacroiliac (SI) joint dysfunction are frequent causes of the chronic lower back pain. Therapeutic solutions include intra-atricular injections with short-term pain relief and surgical fusion, which appears ineffective. Radiofrequency (RF) of the joint capsule or lateral branches has been previously reported with variable successes. Cooling tissue adjacent to the electrode (cooled RF) increases the radius of lesion. We present here the first retrospective data on pain relief and changes in function after such RF denervation. We reviewed electronic records of 27 patients with chronic low back pain (median 5 years) who underwent cooled RF of S1, S2, and S3 lateral branches and of dorsal ramus (DR) L5 following two diagnostic SI joint blocks (>50% of pain relief). Patient sample consisted of 20 women and 7 men, 38 to 92 years old. Pain disability index (PDI), visual analog scale (VAS) pain scores, global patient satisfaction (GPE) and opioid use before and 3-4 months after the procedure were analyzed. One patient had an incomplete chart. Observed were improvements in function (PDI) from 32.7 +/- 9.9 to 20.3 +/- 12.1 (P < 0.001) and VAS pain scores 7.1 +/- 1.6 to 4.2 +/- 2.5 (P < 0.001) at 3-4 months after the procedure. Opioid use decreased from median 30 to 20 mg morphine equivalent. Eighteen patients rated their improvement in pain scores using GPE as improved or much improved, while eight claimed minimal or no improvement. The majority of patients with chronic SI joint pain experienced a clinically relevant degree of pain relief and improved function following cooled RF of sacral lateral branches and DR of L5 at 3-4 months follow-up.