122 Aspergillus strains were isolated from respiratory specimens from 80 patients. Aspergillus fumigatus was the most common species, constituting 88% of the isolates. Susceptibility testing by the NCCLS broth macrodilution procedure revealed that the minimal inhibitory concentration for 50% of the strains (MIC50) was 0.25 mg/l for itraconazole and 0.5 mg/l for amphotericin B. The MIC90 was 1 mg/l for both drugs. To our knowledge, no cases of in vitro resistance during long-term itraconazole use in treatment of Aspergillus infection have been documented. We identified 3 patients infected with A. fumigatus strains that acquired in vitro resistance to itraconazole during prolonged therapy. This finding supports the importance of susceptibility testing.