The skeleton is the most common site for distant metastasis in patients with cancer. To detect bone metastasis and evaluate the efficacy of treatment, we usually use bone scintigraphy and check serum alkaline phosphatase (ALP). However, such evaluation is sometimes difficult due to flare phenomenon. A 61-year-old male was referred to our department with a suspected diagnosis of lung cancer. Following thorough examinations, he was diagnosed with primary lung cancer (adenocarcinoma, Stage IV) and found to have a mutation in the epidermal growth factor receptor gene at exon 21 (L858R). After initiating treatment with oral gefitinib, ALP increased and peaked at 3,592 U/L by 3 weeks and decreased thereafter. At 4 weeks following treatment initiation, bone scintigraphy revealed a marked increase in abnormal accumulation of (99m)Tc-polyphosphate, but the primary tumor and metastases in regions other than the bone were reduced. At 9 weeks after treatment initiation, abnormal accumulations was improved in bone scintigraphy, and computed tomography revealed osteoblastic changes consistent with the accumulated lesion observed by bone scintigraphy. After initiating cancer treatment for bone metastasis, it is not uncommon to observe transient asynchronous accumulation in bone scintigraphy or transient increases in ALP in patients who ultimately respond to the treatment. These changes are called flare phenomenon, and documented in patients with prostate cancer or breast cancer receiving treatment. When determining the efficacy of treatments that target carcinomas with bone metastases, it is important to note that flare phenomenon is often indistinguishable from disease progression indicators.