ABSTRACT We susceptibility tested 17 clinical isolates of Aspergillus fumigatus , for most of which MICs of itraconazole were elevated (MIC at which 50% of the isolates tested are inhibited, 16 μg/ml), against itraconazole, posaconazole, ravuconazole, and voriconazole. Posaconazole was the most active against itraconazole-susceptible isolates. A complex pattern of cross-resistance and hypersusceptibility was seen with voriconazole and ravuconazole, suggesting marked differences in activity and mechanisms of resistance.