The aim of this report was to describe the clinical-pathological data of a case of clinical mastitis in a 20-day postpartum Holstein cow who presented udder edema and rupture of the ventral subcutaneous abdominal vein, and then died. This animal showed an increased volume of the left iliac fossa region, an increase in the size of the udder, and a marked decrease in milk production. Signs of parenchymal inflammation were observed during clinical examination. The screened mug test and California Mastitis Test (CMT) were then performed, and their results were negative. However, Staphylococcus aureus was isolated from milk and liquid collected by puncture from the ventrolateral region of the abdomen. Signs of bleeding and dehydration were found through blood counts and serum biochemistry. Soon after death, the animal was submitted to necropsy, in which rupture of the mammary vein and intense inflammation with fibrin deposition and detachment of the subcutaneous tissue were observed. Histological examination revealed degeneration, diffuse necrosis, and thrombosis with areas of neovascularization associated with fibrin and cellular debris in the mammary vein.