A 35-year-old housewife was diagnosed with onychomycosis and treated by oral terbinafine, 250mg/day, for 4 months. Clinically all infected nails improved gradually. However, her left great toe-nail was not improved thereafter. At that time the proximal end of the onychomycotic lesion was marked with surgical blade, and terbinafine therapy was continued for four weeks. However, the onychomycotic nail was not improved, and the scratch mark passed by the proximal end of the infected nail. Therefore, we detected the resistance to the therapy and switched the medication to itraconazole 100 mg/day and then another scratch mark was done at the proximal end of the onychomycotic lesion. After another 2 weeks the infected nail went along with the scratch mark distally, and showed clinical improvement. After 8 weeks therapy of itraconazole, she was cured clinically and mycologically. The fungal culture was identified as Aspergillus sydowii.