Parosteal osteosarcoma is a low-grade malignant bone tumor that can undergo dedifferentiation. The aim of this study was to analyze clinico-pathological features associated with clinical outcome in a large cohort of patients. Patients consecutively treated for parosteal osteosarcoma at Rizzoli Orthopedic Institute from 1900 to 2018 were reviewed and analyzed. Clinco-pathological data of 195 patients with parosteal osteosarcoma were analyzed. Age at diagnosis ranged from 9 to 75years (median 31). Median follow-up time was 150months (range 3-720). The most common tumor locations were femur (61.5%), humerus (15.9%) and tibia (12.8%). Wide surgical margins were achieved in 125 (64.1%) patients. Medullary involvement was present in 69 (35.4%) cases. Dedifferentiation occurred in 48 (24.6%) patients. Forty-five patients developed recurrence (23.1%; median time to recurrence of 36months). At last follow-up, 155 (79.5%) patients were alive and without evidence of disease, 8 (4.1%) were alive with active disease, 23 (11.8%) died from disease, and 9 (4.6%) from unrelated causes. Patients with dedifferentiated parosteal osteosarcoma had worse 5-year (65% vs. 96%) and 10-year survival (60% vs. 96%) when compared to conventional tumors (P<.001). Wide surgical margins had positive impact on both disease-free (P<.001) and overall survival (P=.036). Medullary involvement, age at presentation and tumor size had no impact on survival. Dedifferentiation is the most important factor that negatively impacts clinical outcome. Surgical aim is to ensure radical removal with wide surgical margins to improve disease-free survival.