OBJECTIVE: The objective of this study is to assess sildenafil and N-desmethyl sildenafil (DMS) exposure in infants receiving sildenafil for the treatment of pulmonary hypertension (PH). STUDY DESIGN: Data were collected from six infants receiving sildenafil for the treatment of PH and plasma samples were collected at the time of routine laboratory blood draws. The echocardiography results were assessed for improvement in right ventricular (RV) hypertension following sildenafil treatment. RESULT: The median (range) sildenafil and DMS concentrations were 27.4 ng ml(-1) (2.6 to 434.0) and 105.5 ng ml(-1) (3.6 to 314.0), respectively. The median metabolite-to-parent ratio was higher in infants receiving co-medications that can induce cytochrome P450 (CYP) enzymes (5.2 vs 0.7). The echocardiography results showed improvement in RV hypertension for the majority of infants (5/6). CONCLUSION: The concentrations of sildenafil and DMS were within the previously observed ranges. Our results suggest that caution may be warranted when CYP-related co-medications are administered during sildenafil treatment for PH.