Focal myositis is a rare disease associated with inflammatory changes invading the skeletal muscles and may be triggered by nerve lesion. We describe a case of 33-year-old woman who was diagnosed focal denervation myositis of piriformis and gluteal muscles associated with sciatic neuropathy because of severe uterine enlargement due to twin pregnancy misdiagnosed as lumbar herniated nucleus pulposus. MRI is one of the key diagnostic tools in the assessment of focal denervation myositis for differential diagnosis. Nonsteroidal anti-inflammatory drugs are commonly used for treatment of focal myositis. Spontaneous regression is obtained in numerous cases.