Azole resistance is an emerging increasing problem in Aspergillus fumigatus that results in treatment failure. Alternative treatments may improve the therapeutic outcome in patients with azole-resistant invasive aspergillosis (IA). Little is known about the in vivo efficacy of the echinocandin anidulafungin (AFG) in IA. The in vivo efficacy of 2.5, 5, 10, and 20 mg/kg of body weight AFG was assessed against two clinical Aspergillus fumigatus isolates with identical AFG minimum effective concentrations (MECs; 0.03 mg/liter) in a murine model of IA: a wild-type voriconazole (VCZ)-susceptible (VCZ(s)) A. fumigatus isolate (AZN 8196) and a VCZ-resistant (VCZ(r)) A. fumigatus isolate (V52-35) harboring the TR(34)/L98H resistance mechanism (substitution at codon L98 in combination with a 34-bp tandem repeat in the promoter region of the CYP51A gene). The pharmacokinetics of AFG were also assessed for each dose. Increasing doses increased survival for both isolates in a manner dependent on the AFG dose level (R(2) = 0.99 and 0.95, respectively) up to a maximum of 72.7% and 45.45% for the VCZ(s) and VCZ(r) isolates, respectively. The area under the concentration-time curve (AUC) correlated significantly with the dose in a linear fashion over the entire dosing range (R(2) = 0.86). The Hill equation with a variable slope fitted the relationship between the 24-h AUC/MEC ratio and 14-day survival well (R(2) = 0.87; P < 0.05). The 50% effective AUC/MEC for total AFG was 126.5 (95% confidence interval, 79.09 to 202.03). AFG treatment improved the survival of mice in a dose-dependent manner; however, a maximal response was not achieved with either isolate even in those treated with the highest AFG dose.