A 45-year-old house-wife, who had undergone surgical treatment for lung adenocarcinoma 2 years previously, presented with a left renal mass detected by follow-up CT. During 7 months of observation, it rapidly enlarged from 1 cm to 3 cm in maximum diameter. Although the short doubling time (40 days) suggested that the tumor was metastatic, abdominal urtrasonography, angiography and CT scan did not exclude primary renal cell carcinoma. Left nephrectomy was performed. Histologically, the tumor proved to be metastasis from the previous lung adenocarcinoma. She has been alive with pulmonary recurrence 13 months after nephrectomy. The doubling time of the tumor may be one of the key factors in differentiating a metastatic renal tumor from a primary carcinoma.