This study investigated the epidemiology and prognostic significance of mitral valve prolapse, detected by ultrasonography, in 153 cavalier King Charles spaniels which were screened consecutively during a period of one year. Seventy-five of the dogs, which had either no murmur or a grade I murmur on screening, were reexamined three years later. The screening revealed that 82 per cent of the dogs aged one to three years and 97 per cent of the dogs over three years had various degrees of mitral valve prolapse. The presence and severity of the condition were independent of gender but correlated positively with age and negatively with bodyweight. The degree of mitral valve prolapse at screening correlated with the regurgitation status (murmur intensity and size of the regurgitant jets) at re-examination and with the percentage increase in the left ventricular end diastolic diameter over the three-year period. The presence of a grade I murmur was not a useful prognostic indicator.