The effect of left-sided valvular regurgitation (LSVR) on the mortality of middle-aged and older horses was investigated in a prospective cohort study involving 19 yards and 1153 horses. The horses were examined to determine whether they had a cardiac murmur and its type, and their age, sex, breed type and occupation were recorded. They were followed up at intervals of two years by postal questionnaire, and after four years information on 773 horses was available. There was no significant difference in the mortality of the horses with and without LSVR, but small horses had a significantly higher risk of having LSVR than small ponies (odds ratio [OR] 2·33), and older horses were slightly more likely to have LSVR than young horses (OR 1·07). Twenty-nine per cent of the deaths reported by the owners were due to orthopaedic problems, 23·3 per cent to gastrointestinal problems, and only 7·9 per cent to cardiovascular problems. Orthopaedic problems were the main cause of death in the horses, and gastrointestinal problems were the main cause of death in the ponies.