A fracture of the humeral shaft can occur by direct or indirect injury. Most occur as a result of direct injury mechanisms such as falls from a height, direct blows, and traffic accidents. Recently, the population enjoying watersports for leisure is increasing and 'flyfish riding', in which passengers ride an inflatable raft drawn by a motorboat, may cause humeral shaft fracture as twisting and axial compression forces occur on the humeral shaft while boarding. Accordingly, the incidence of humeral shaft fracture is expected to increase as more people are expected to enjoy leisure sport activities such as 'flyfish riding'. We report 4 cases of humeral fracture that occurred during this activity in the year 2011.