Polymyositis (inflammatory disease of the muscles) is classified as follows:
| Form | Frequency |
| Primary idiopathic (without identifiable cause) polymyositis | 34 % |
| Primary idiopathic dermatomyositis* | 29 % |
| Polymyositis/dermatomyositis associated with malignant tumors (concomitant disease of cancer) | 9% |
| Polymyositis/dermatomyositis with vasculitis (inflammation of blood vessels) in childhood | 7 % |
| Polymyositis/dermatomyositis in collagenoses (overlap-group/overlap syndromes). | 21 % |
* Dermatomyositis = polymyositis involving the skin.
