An 8-year-old Hispanic boy with a hypoplastic left thumb, absent right thumb, and short stature experienced right leg pain and limp. A right tibial lesion was imaged and found to be osteosarcoma on biopsy. A 6-year-old Hispanic girl with congenitally absent thumbs experienced a pathologic fracture of her left femur after a minor sports injury. The radiologic abnormality seen was diagnosed as osteosarcoma on biopsy. Both patients continue to do well after intensive preoperative and postoperative high-dose chemotherapy and definitive reconstructive limb surgery. Osteosarcoma has been linked to several congenital syndromes in which absent thumbs are a feature. These two patients with absent thumbs and no definable syndrome experiencing osteosarcoma suggest that congenitally absent thumbs might be a risk factor for osteosarcoma in the absence of a syndrome.