A national study of exercise related morbidity (ERM) in England and Wales was carried out using a postal questionnaire sent to 28,857 adults aged 16-45 years. The questionnaire asked about regular participation in sports or other recreational fitness activities involving physical exercise, and for details of any injuries occurring during a 28 d reference period. A return rate of 68% was achieved. Comparisons with other national data sources indicated that the information obtained was reliable. It is estimated that each year there are 29 million incidents resulting in new or recurrent injuries, however minor, of which 9.8 million (95% confidence interval 8.1 to 11.4 million) result in new 'substantive' injuries which are potentially serious, result in treatment, or in participants being unable to take part in their usual activities. Soccer accounted for more than 25% of all ERM, but the risk of a substantive injury in rugby was three times that in soccer. Over one third of ERM occurred in men aged 16-25 years. The most frequently reported injuries were sprains and strains of the lower limbs. Treatment was sought in approximately 25% of ERM incidents and 7% of all new ERM incidents involved attendance at a hospital accident and emergency department. The treatment provider most likely to be consulted was a general practitioner, but physiotherapists and complementary medicine practitioners were also consulted frequently. To maximize the health benefits of exercise, research strategies to reduce the volume and severity of ERM and to identify the most appropriate ways of managing ERM should be set.