Fifteen dogs with nasal aspergillosis were treated with ketoconazole (5 mg/kg of body weight, q 12 h, PO) for 2 to 18 weeks. Four dogs whose conditions deteriorated during treatment received ketoconazole for less than the prescribed 6 weeks. Six months or more later, only 47% of the dogs were determined to be disease-free, on the basis of no fungal growth on culture. It was concluded that ketoconazole at this dosage is a useful treatment for canine nasal aspergillosis, but is no more effective than thiabendazole.