Tarlov cysts were first described in 1938 as an incidental finding at autopsy. The cysts are usually diagnosed on MRI, which reveals the lesion arising from the sacral nerve root near the dorsal root ganglion. Symptomatic sacral perineural cysts are uncommon and it is recommended to consider Tarlov cyst as a diagnostic of exclusion. We report a case of a patient with voluminous bilateral L5 and S1 Tarlov cyst, and right hip osteonecrosis to increase the awareness in the orthopaedic community. A 57-year-old female, in good health, with chronic low back pain since 20 years, presented suddenly right buttock pain, right inguinal fold pain and low back pain for two months, with inability to walk and to sit down. X-ray of the lumbo-sacral spine revealed asymmetric discopathy L5-S1 and L3-L4. X-ray of the right hip did not reveal anything. We asked for an MRI of the spine and it revealed a voluminous fluid-filled cystic lesion, arising from the first sacral nerve root on both side and measuring 3,3cm in diameter. The MRI also show a part of the hip and incidentally we discovered an osteonecrosis Ficat 3 of the right femoral head. The patient was taken for a total hip arthroplasty, by anterior approach. Patient appreciated relief of pain immediately after the surgery. The current case show that even if we find a voluminous cyst we always have to eliminate other diagnosis (especially the frequent like osteonecrosis of the femoral head) and mostly in the case of unclear neurological perturbation.