OBJECTIVE The aim of the study was to define how many whiplash injuries occur in Finland in traffic accidents and the degree of severity of these injuries using the whiplash-associated disorders (WAD) classification presented by the Quebec Task Force, and to define possible long-term health effects caused by whiplash injury as well as the duration of whiplash-associated sick-leaves. METHODS This was a prospective one-year-follow-up study. Fourteen insurance companies paving compensations for traffic accidents in Finland sent the accident reports and medical certificates of all neck injuries attributable to traffic accidents to the research team. The material was collected from neck injuries that had occurred in traffic accidents during the year 1998. RESULTS The majority of those suffering a whiplash injury were women. On the basis of the WAD classification, most whiplash injuries were mild, belonging to grades WAD I and II. At one year from the accident nearly 10% considered that their health had been impaired significantly as a result of their neck injury. Over 10% of those questioned had been on sick-leave for over a month but only 1.5% had been on sick-leave associated with the injury for more than 6 months. The most common symptom after one year was neck pain or neck pain combined with headache and symptoms in the upper extremities. No major changes related to the seasons of the year were found. CONCLUSIONS The number of reported neck injuries in proportion to all traffic accidents involving physical injuries is small, even in proportion to rear-end collisions. In a considerable proportion of collision patients, whiplash injury does result in significant impairment which can last as long as a year after the accident. The WAD classification predicts the duration of work disability and the long-term health damage caused by the injury. Since the appearance of symptoms and the individual need for rehabilitation due to impaired functional capacity do not depend solely on the tissue damage and biomechanical forces involved in the collision, in the future it will be important to determine which factors are responsible for the differences in coping after a collision.