Spindle-cell sarcomas are a group of aggressive malignant soft-tissue tumors with diverse clinical presentations. While some of these tumors may represent de novo malignant transformation of benign entities, others may present with a long indolent course before their invasive nature is apparent. We report the case of a previously healthy 73-year-old female with spindle-cell sarcoma of the left popliteal fossa who initially presented with a painless mass of several months' duration. Magnetic resonance imaging (MRI) suggested a benign vascular lesion/venous angioma, but after the patient's clinical course changed, repeat MRI 18 months later was consistent with an infiltrative soft-tissue malignancy/sarcoma. Excisional biopsy revealed a stage II spindle-cell sarcoma. The patient then underwent radiotherapy and subsequent above-the-knee amputation. Three years following the amputation, the patient is free of local tumor recurrence or metastatic disease.