Two-hundred sequential Aspergillus fumigatus isolates recovered from 26 immunocompromised patients with invasive aspergillosis or bronchial colonization were tested for their in vitro susceptibility to posaconazole, itraconazole, voriconazole, terbinafine and amphotericin B. Twenty-one patients were treated with amphotericin B and/or itraconazole. Antifungal susceptibilities of the isolates recovered before treatment were not significantly different from those of isolates recovered after the onset of antifungal therapy. The highest MICs were 0.125, 0.5, 0.5, 1 and 1 microg ml(-1) for posaconazole, itraconazole, voriconazole, terbinafine and amphotericin B, respectively. It is concluded that the emergence of resistance in A. fumigatus during antifungal therapy with amphotericin B or itraconazole is an uncommon phenomenon.