Hamate hook fractures are rare injuries but appear to occur frequently in underwater rugby, the reason for which was investigated in this study. High-level underwater rugby players with hook fractures diagnosed during a five-year interval (2005-2010) were studied retrospectively. Medical data on these patients were reviewed for information on the mechanism of injury, type of fracture, radiological imaging, treatment, and outcome. In ten patients, hook fractures of the leading hand were confirmed by computed tomography, all of which were associated with specific injuries during underwater rugby games. Conservative treatment resulted in delayed healing or non-union, wherefore fragment excision and open reduction and internal fixation was performed in ten and five patients, respectively, while two patients declined surgery. After surgery, all patients were able to play underwater rugby again. In underwater rugby, hook fractures occur frequently due to high and repeated forces applied to the leading hand during games.