The authors present a case of a 12-year-old boy with a known family history of multiple hereditary exostoses who presented with knee swelling. Physical exam ination and subsequent workup revealed a pseudoaneurysm associated with an osteo chondroma. We present this interesting case and review the literature. The authors present a case of a 12-year-old boy with a known family history of multiple hereditary exostoses who presented with knee swelling. Physical exam ination and subsequent workup revealed a pseudoaneurysm associated with an osteo chondroma. We present this interesting case and review the literature.