The differential diagnosis of low back pain is long and rarities are under-diagnosed, with problems generally simplified as lumbar vertebral spondylosis or rheumatic conditions. Abscesses of piriform muscle are a particularity worth of evaluating when specific MRI changes are detected, and the condition might be underdiagnosed leading to delays in the treatment. We describe the case of an 18-year-old male with pyomyositis of left piriform muscle, complicated with iliac and femoral vein thrombosis, that responded well to combined antibiotherapy, anticoagulants and drainage.