Background: This article reviews our 47-year experience of radiotherapy (RT) as adjuvant treatment for histologically confirmed pigmented villonodular synovitis (PVNS). Methods: We identified seven women and two men with diffuse PVNS of the knee (n = 8) or hip (n = 1) who were treated with postoperative RT between 1969 and 2015 at the University of Florida. The median patient age was 59 (range, 39-79) years. The median follow-up was 5.5 (range, 2.1-26.0) years. All patients received megavoltage external-beam RT using three-dimensional conformal techniques. The median RT dose was 36 (range, 19-45) Gy in a median of 18 (range, 13-36) fractions; four patients received twice-daily fractionation, and five once daily. Results: Of the nine patients, five had clinically and / or radiographically stable disease or were disease-free; three failed to achieve local control; and one had a questionable local recurrence based on magnetic resonance imaging 4 months later. None had acute or long-term complications from RT. Conclusion: Synovectomy and watchful waiting remains the primary management for PVNS. In cases of extensive disease with incomplete excision and / or local recurrence, local RT should be considered.