The aim of the present study was to explore whether polymyositis may be considered as an isolated, organ-specific disease or more suitably as a secondary or associated entity. A retrospective re-evaluation of all the muscle biopsies performed at the Hospital Clínic of Barcelona showing histopathological pattern of polymyositis from January 1997 to May 2012 was carried out. The medical records of the patients with the aforementioned pathological pattern were also reviewed. From 1.290 muscle biopsies performed during the period evaluated, 36 with polymyositis pattern were identified. At the time of muscle biopsy, polymyositis pattern was secondary or associated with other disease in 26 patients and was classified as isolated in the remaining ten patients. After pathological re-evaluation and long-term clinical follow-up, only one patient remained with this diagnosis. Overall, the main final diagnosis related to the initial polymyositis pattern was inflammatory myopathy associated with connective tissue disease. Several other associated conditions were also identified. Isolated polymyositis is highly uncommon. Ruling out potential associated or confusing entities, like inclusion body myositis, overlap syndromes, infections, and cancer, is mandatory.