Dermatomyositis is associated with an underlying malignancy in about 24% of cases. This association is also true for polymyositis but is less prominent. The malignancy is usually an adenocarcinoma of the ovary, lung, or gastrointestinal tract in Western countries and nasopharyngeal carcinoma in Southeast Asia, Southern China, and Northern Africa. Factors predictive of malignancy in myositis patients include more severe skin and muscle disease and the absence of overlap connective tissue disease features, such as interstitial lung disease. Anti-p155/140 antibodies have a strong predictive value for malignancy in adult patients. Patients with dermatomyositis or polymyositis require an evaluation for occult malignancy at the time of diagnosis and, in some cases, in the event of a subsequent recurrence. This paraneoplastic phenomenon may stem from an immune reaction to antigens expressed in both cancer cells and regenerating fibers in affected muscle.