The authors express their sincere gratitude to orthopaedic surgeons listed in the following who participated in this study registering, performing hip arthroplasties on, following up the patients with nontraumatic osteonecrosis of the femoral head, and collecting the data. Institutions (researchers [currently working at other hospitals]) in geographical order from north to south: Asahikawa Medical University (Hiromasa Tanino), Hokkaido University (Tomohiro Shimizu, Takuji Miyazaki, Takuya Ogawa), Sapporo Medical University (Ima Kosukegawa), Yamagata University (Juji Ito), Chiba University (Shigeo Hagiwara, Yuya Kawarai), Dokkyo Medical University Saitama Medical Center (Gaku Koyano, Ryota Shinada, Tetsuya Tachibana, Moe Suzuki), The University of Tokyo (Takeyuki Tanaka, Naoto Kaminaga), Institute of Science Tokyo (Kazumasa Miyatake), Tokyo Medical University (Takaaki Shishido, Toshinori Masaoka, Toshiyuki Tateiwa, Tsunehito Ishida), Yokohama City University (Hyonmin Choe, Hiroyuki Ike), Showa University Fujigaoka Hospital (Tsubasa Ishikawa, Itaru Kachi), Shinshu University ([Hiroyuki Kodaira], Hiroshi Horiuchi), Kanazawa Medical University (Toru Ichiseki), Nagoya University (Yusuke Osawa), Mie University (Akihiro Sudo, Yohei Naito), Kyoto University (Yutaka Kuroda, Toshiyuki Kawai, Yaichiro Okuzu, Yugo Morita), Osaka University (Kazuma Takashima, Keisuke Uemura, [Masaki Takao]), Osaka National Hospital (Makoto Kitada, Yoshichika Hashimoto, Makoto Iwasa), Kansai Rosai Hospital (Tsuyoshi Koyama), Osaka Metropolitan University (Ryo Sugama, Shigekazu Mizokawa, Yukihide Minoda), Hiroshima University (Hideki Shozen, Shinichi Ueki, Hiroki Kaneta), Yamaguchi University (Tomoya Okazaki, Yuta Matsuki, Takehiro Kaneoka, Takehiro Kawakami, Kazuhiro Yamazaki), Ehime University (Masaki Takao), Kyushu University (Yasuharu Nakashima, Ryosuke Yamaguchi, Takeshi Utsunomiya, Noriko Yamamoto, Hidenao Tanaka, Yusuke Ayabe, Kosei Sakamoto), Fukuoka University (Fumihiro Yoshimura, Yoshiaki Hideshima, Taiki Matsunaga, Hajime Seo, Takuaki Yamamoto), Saga University (Masaaki Mawatari, Masanori Fujii, Masaya Ueno), Nagasaki University (Ko Chiba, Kyosuke Kobayashi, Kazuteru Shiraishi), Miyazaki University (Takero Sakamoto, Yoichiro Yamaguchi), and University of the Ryukyus (Masamichi Onaga, Takahiro Igei, Fumiyuki Washizaki).