A 14-year-old girl with cerebral palsy presented for bilateral lower limb surgery for spasticity. A lumbar epidural catheter was sited under general anaesthesia on the third attempt and used intra-operatively with good effect. A local anaesthetic infusion was used for postoperative analgesia but was noted to be leaking under the dressing with a patchy, unilateral block. The catheter was therefore removed on the second postoperative day. Following discharge, the patient progressively developed new back and leg pain for which she was re-admitted seven weeks later. This was investigated and initially thought to be myositis of the erector spinae muscles on magnetic resonance imaging. When the patient failed to respond to treatment, a muscle biopsy demonstrated desmoid fibromatosis. Trauma may cause or accelerate the development of desmoid fibromatosis, which has also been theorised to arise from scar tissue in previously injured areas. We hypothesise that challenging epidural placement or the leakage of the local anaesthetic agent into the surrounding muscular tissue, inducing local myonecrosis, could have been the triggering or accelerating event in tumour development. This may be the first reported case of extra-abdominal desmoid fibromatosis in association with epidural placement.