A 8 kg, 3-year-old male West Highland white terrier dog with a 1.5-year history of chronic, severely pruritic, seborrheic skin disorder was referred to the Veterinary Medical Teaching Hospital of the Tokyo University of Agriculture and Technology. On physical examination, lesions were observed on entire cutaneous surface, except for face, dorsum of body, and footpads. Skin lesions were characterized by diffuse erythema, erythematous papules, severe alopecia, hyperpigmentation, and lichenification. Tape strip tests of skin lesions revealed cocci and Malassezia infections. The intradermal allergy tests revealed positive reactions to Japanese cedar pollen, but the non-seasonal clinical signs were not compatible with atopic dermatitis caused by this pollen. Results of hematological examination, serum chemistry and thyroid gland profile were normal. Examination of skin biopsy exhibited hyperplastic superficial perivascular dermatosis with severe acanthosis, excessive keratinocyte mitoses, patchy or diffuse mild spongiosis, and lymphocytic exocytosis in epidermis. Perivascular to interstitial mononuclear cells infiltration was seen in the superficial dermis. Based on the results of examination described above, epidermal dysplasia was diagnosed. Treatments with administration of antibiotics, etretinate, and prednisolone orally combined with topical ketoconazole cream and antiseborreic shampoos had no good results. Following treatment with long-term oral itraconazole at 10 mg/kg daily and chlorhexidine shampoos was successful. However, when itraconazole therapy was stopped, the condition worsened twice within 2 or 3 months. Readministration of itraconazole produced improvement within 4 weeks. This dog has now been controlled periodical itraconazole therapy.