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The development of a second primary malignancy in a patient with a preexisting diagnosis of metastatic cancer may be easily overlooked or misattributed to progression of disease. We report 3 patients with clear-cell renal cell carcinoma (RCC) metastatic to the lungs who were subsequently diagnosed with non–small-cell lung cancer (NSCLC). We examined the frequency of this occurrence within our institution and report on the radiographic findings that may help distinguish between metastatic RCC and primary lung cancers.Patients who received systemic targeted therapy for metastatic RCC at our institution between January 2006 and October 2013 were identified, and the proportion and incidence rate for developing NSCLC with preexisting metastatic RCC were calculated.Two percent (3/151; 95% confidence interval [CI], 0.68%-5.68%) of patients treated for metastatic RCC with systemic targeted therapies at our institution were subsequently diagnosed with NSCLC, increasing to 3.5% (3/85; 95% CI, 1.21%-9.87%) among patients with known RCC pulmonary metastasis. The incident rate for development of NSCLC in patients with metastatic RCC was 0.87 per 100 person-years (95% CI, 0.22-2.4).The subsequent diagnosis of a primary lung cancer in metastatic RCC patients occurred in 2% of patients at our institution and is underreported in the literature. Primary NSCLC may be underdiagnosed in patients with metastatic RCC. Both the radiographic appearance and clinical behavior of a lesion may hold clues that can help distinguish between a new primary and progression of metastatic disease.The development of a new primary lung cancer in patients treated for metastatic renal cell carcinoma is likely underappreciated. We examine the frequency of this occurrence at our institution, report 3 cases in detail, and describe the clinical and radiographic features that help distinguish between metastatic and primary lung cancers.