Parallel administration of the proton pump inhibitor (PPI) esomeprazole has been shown to decrease oral bioavailability of posaconazole in healthy volunteers. We prospectively analyzed serum samples (n = 59) obtained from hematology patients (n = 27) under posaconazole prophylaxis. Patients treated concomitantly with pantoprazole had significantly lower posaconazole levels than patients without PPI treatment (median levels of 630 μg/liter versus 1,125 μg/liter, respectively). These results suggest that drug monitoring is relevant when posaconazole and pantoprazole are administered concomitantly. Copyright © 2010, American Society for Microbiology. All Rights Reserved.