During a 1-year period, data on all acute injuries treated at emergency departments were entered into a registry; of these injuries, 6% involved the knee joint. The patients were followed until a definite diagnosis was established. The rate of anterior cruciate ligament injuries and meniscus tears was, respectively, 0.3 and 0.7 (injuries per 1,000 inhabitants per year). The rate of dislocations of the patellofemoral joints was equal to the rate of anterior cruciate ligament injuries. Ten percent of the cases involved ligament injuries, and isolated ruptures of the anterior cruciate ligaments were twice as common as combined lesions of this ligament. Injuries of both the collateral ligament and the meniscus represented 6% of the cases. In most age groups male patients predominated, but the frequency of anterior cruciate ligament injuries was similar in both sexes, while the frequency of collateral ligament injuries and meniscus tears varied according to both gender and age. Only 27% of the injuries were associated with sports activities, but they were found twice as often among athletes than people injured in nonathletic accidents. Ruptures of the collateral ligament and anterior cruciate ligament were four and seven times more common among athletes, respectively, while athletes sustained fewer meniscus tears than people involved in nonathletic activities. Half of the patients needed further care after primary treatment, and 18% were primarily (65%) or secondarily (35%) admitted to in-patient care. Operative treatment, including arthroscopic examination, was performed in 20% of cases. Minor ligament sprains needed further treatment in half of the cases and 7% of patients with minor sprains underwent arthroscopic examination, while major ligament injuries and meniscus tears accounted for half of all knee injuries admitted for in-patient care. This prospective analysis of all acute injuries in the population treated at hospitals shows epidemiologic aspects that previous investigations of selected data on knee injuries have missed.