Forty-seven patients presented during a 1-year period with fractures of the extremities produced by the impact of a baseball bat. This represented an increase of fourfold in fractures associated with baseball bat trauma as compared with the previous year. Those patients with tibia fractures were retrospectively reviewed in terms of associated soft-tissue injuries, fracture pattern, and fracture healing. Potential force generated and kinetic energy transferred when a baseball bat strikes the tibia were calculated mathematically to provide correlation with clinical observations. Tibia fractures produced by baseball bats represented 10% of all tibia fractures presenting from July 1990-July 1991. Of these 11 tibia fractures, 3 developed compartment syndrome, necessitating fasciotomy and subsequent skin grafting, and 1 developed osteomyelitis, which eventually resulted in amputation. One patient had superficial skin blistering that required a delay in definitive fracture care of 11 days and subsequent skin grafting. There was one nonunion and three delayed unions requiring additional operative intervention. This incidence of compartment syndrome (27%) is nine times higher than the overall incidence, at this institution, of compartment syndrome (3%) in tibia fractures produced by other mechanisms. The maximum potential kinetic energy produced during the impact of a baseball bat is 515 kg/m2/s2: 25% higher than that produced by a 9-mm bullet fired at a distance of 6 ft (407 kg/m2/s2). The potential force transmitted from a bat to the tibia at the time of collision is 8,000 lb, three times that of the bullet (626 lb).(ABSTRACT TRUNCATED AT 250 WORDS)