A 12-year-old girl with cystic fibrosis was diagnosed with a high-grade glioma after radiographic and biopsy results confirmed the primary intracranial lesion. She was treated with single-agent erlotinib during and after daily localized radiation therapy. Pharmacokinetic studies were conducted to assess the effect of pancreatic enzyme deficiency and intestinal malabsorption secondary to cystic fibrosis on the bioavailability of orally administered erlotinib, a lipophilic drug. Pharmacokinetic analysis of plasma samples from days 1 and 8 demonstrated that absorption of oral erlotinib was not affected by the patient's cystic fibrosis when the drug was given concomitantly with pancreatic enzyme replacement. When pediatric patients with cystic fibrosis are receiving erlotinib or other lipophilic oral drugs, continued supplementation of pancreatic enzymes is recommended, with therapeutic drug monitoring of plasma drug concentrations when feasible, and close observation for therapeutic responses and adverse effects.