Pseudomalignant heterotopic ossification is a rare, self limited connective tissue disorder of unknown origin that may occur atypically during childhood and can simulate either soft tissue sarcoma or fibrodysplasia ossificans progressiva. A complex constellation of diagnostic features usually enable the differentiation of pseudomalignant heterotopic ossification from extraosseous osteosarcoma and fibrodysplasia ossificans progressiva during a time span of approximately 8 to 12 weeks. Orthopaedic surgeons who treat children with connective tissue tumors should be familiar with pseudomalignant heterotopic ossification and its differential diagnosis. The occasional mild and variable expression of fibrodysplasia ossificans progressiva rarely may make it more difficult to distinguish from pseudomalignant heterotopic ossification. It is possible that pseudomalignant heterotopic ossification is a form fruste of fibrodysplasia ossificans progressiva.