Pigmented villonodular synovitis (PVNS) of the ankle is a very rare, locally aggressive, proliferative disorder. Although surgical excision represents the standard curative treatment, the PVNS relapse rate is high. We present our study of five young athletes (range 20-36 years) with a histopathological diagnosis of PVNS of the ankle, who were treated by surgery and adjuvant radiosynoviorthesis (RSO). The operation involved either arthroscopic (four patients) or open (one patient) debridement, followed by intraarticular RSO with the radiopharmaceutical erbium-169 (Er). They were evaluated with the Foot Function Index (FFI) and a visual analog scale (VAS) for pain. At a median follow up period of 47 months (range 36-54 months), all five patients reported marked pain relief with improvements in their daily activities. In particular, the median FFI decreased from 77% (range 71.0%-84.5%) pre-treatment, to 0.5% (range 0%-6%) after treatment. The median VAS score decreased from 4 (range 3-7) to 0 (range 0-1), respectively. Throughout the follow-up period, there were no major complications regarding either therapeutic intervention (arthroscopic or open debridement, RSO). Based on these results, it can be concluded that adjuvant RSO with Er following surgical excision is effective and safe in the treatment of PVNS of the ankle.