INTRODUCTION: Drugs directed toward the epidermal growth factor receptor (EGFR), such as erlotinib (Tarceva®) and gefitinib (Iressa®), are used for the treatment of patients with advanced non-small cell lung cancer (NSCLC), including patients with brain metastases. However, whether erlotinib actually enters into brain metastases has not been adequately elucidated. In this study, we investigated the accumulation of [¹¹C]-erlotinib by positron emission tomography (PET) combined with computed tomography (CT) and magnetic resonance imaging (MRI). METHODS: A 32-year-old patient with NSCLC and multiple brain metastases was treated with first-line erlotinib. EGFR mutations were determined by analyzing a fine-needle lung tumor biopsy taken before the treatment. A PET/CT of the brain with [¹¹C]-erlotinib was performed during treatment, and a MRI of the head and a CT of the chest were performed pre- and posttreatment. RESULTS: The primary lung tumor displayed an erlotinib-sensitizing exon 19 deletion in the EGFR gene, and [¹¹C]-erlotinib PET/CT showed accumulation in the brain metastases. Posttreatment MRI and CT demonstrated regression of both brain metastases and primary lung tumor. CONCLUSION: Our data demonstrated that erlotinib accumulated in brain metastases in a NSCLC patient who responded to the treatment.