Posaconazole, a triazole antifungal agent, displays a highly variable pharmacokinetic profile due in part to the oral solution, the only currently available dosage form. Posaconazole activity appears to be dose-dependent and clinical outcomes correlate with steady-state serum trough concentrations and area under the concentration curve (AUC) over minimum inhibitory concentration (MIC) ratios. Therapeutic drug monitoring (TDM) has been recommended to help overcome the unpredictable nature of the drug. The impact of obesity of posaconazole pharmacokinetics and the use of TDM in this population has not been well described; however, limited data demonstrate increased weight correlates with lower serum concentrations and reduced drug exposure. We describe the use of TDM in an obese man, 12 months status post hematopoietic stem-cell transplant (HSCT), receiving posaconazole therapy for a presumed invasive fungal infection (IFI).