The patient, an 11 year old Quarter Horse gelding, was presented to the Cornell Equine and Farm Animal Hospital in the fall on referral for suspected Clostridium botulinum infection. The patient had a one day history of exercise intolerance, and acutely developed hyporexia, dysphagia, dull mentation, and muscle fasciculations the morning of presentation. The patient was presented with clinical signs consistent with a severe, acute myopathy, including skeletal muscle weakness, muscle fasciculations, and dark brown urine. The most significant of the patient’s laboratory abnormalities were markedly elevated creatine kinase and asparate aminotransferase. Despite supportive therapy, the patient progressed to recumbency within 12 hours of presentation and euthanasia was elected. Necropsy and histopathology revealed findings consistent with seasonal pasture myopathy, including severe, acute, myonecrosis and pigment nephropathy. The results of acylcarnitine and urine organic profiles were received post-mortem and were diagnostic for seasonal pasture myopathy.