This report describes the presentation, diagnostic evaluation, and treatment of a 4 year old late lactation Jersey cow that presented to the Emergency Service at Cornell Farm Animal Hospital with acute, severe clinical mastitis which cultured pure growths of Klebsiella pneumoniae. This bacterial species along with Escherichia coli are coliforms and are a common cause of mastitis in dairy cattle. Therefore, the etiology, clinical signs, prognosis, treatment, and prevention of coliform mastitis are described in depth with comparisons between the mild and severe form of this disease.