A 35-year-old woman of Indian origin presented with a 5-month history of lower back pain, radiating down the back of her right leg in distribution of the sciatic nerve. Referral was made to the spinal clinic querying sciatica, but a deterioration in her symptoms developed, and the patient presented to the Accident and Emergency department. She was significantly tender at right sacroiliac joint and had positive psoas sign. The MRI scan showed a large iliopsoas abscess causing bony destruction, and extended culture was positive for mycobacterium tuberculosis. The patient was initially diagnosed with sciatica yet had a positive psoas sign and a painful sacrum. It is important that primary physicians are aware of the relations the iliopsoas muscle and the potential effect an abscess can have here. A sinister underlying cause of a patient's sciatic distribution of pain should be excluded before accepting a diagnosis of mechanical back pain.