Summary Four out of 12 horses grazing a field in Berkshire, England, suffered a prostrating illness and died within 12 to 72h. Serum biochemical abnormalities, including markedly elevated muscle enzymes, were demonstrated and at post mortem widespread myodegeneration was found in both skeletal muscle and myocardium. Urine analysis revealed myoglobinuria, and renal changes were seen histologically. Although similar pathologically, the clinical syndrome and circumstances of the outbreak were not typical of equine exertional rhabdomyolysis (EER). The outbreak bore a striking resemblance to other reported sporadic outbreaks of an atypical myoglobinuria occurring in grazing horses. A number of potential aetiological and contributory factors (including herbicide toxicity) were considered, but the aetiology remains unresolved.