A female 7 years old Labrador retriever was attended with a severe dermatological disease. The dog was treated with cephalexin (25 mg/kg/12 h/10 days), chlorhexidine shampoo and nutraceuticals (Omega 3, vitamins and minerals), with no clinical improvement. The clinical and histopathology results suggested a diagnosis of mycosis fungoides. Accordingly, treatment schedule was changed, but the dog did not show any clinical remission after several days under treatment and died. Immunohistochemistry evidenced a severe infiltration of CD3 T-cells collected around the basal squamous epithelia. Immunohistochemistry results confirmed the first mycosis fungoides case report in Colombia. Clinical findings and the most relevant treatment schedules are discussed.