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The purpose of this study was to establish the contemporaneous frequency of metastases within the kidney as opposed to primary renal tumors in patients with an active primary nonrenal malignancy and to identify the differentiating features.We retrospectively identified all patients with an active primary nonrenal malignancy (group 1) who had also undergone at least 2 contrast-enhanced abdominal CT examinations spaced 1 year apart. The radiologic and pathologic data of these cases were reviewed and the incidence of metastasis to the kidney versus primary renal tumors established. These data were compared with a separate group who presented with primary renal malignancy from the outset (group 2).In the study were 2340 patients with primary nonrenal malignancy (group 1) and 231 patients with a primary renal malignancy (group 2). For group 1, the mean age was 63 years and 51% were men; for group 2, the mean age was 59 years, and 58% were men. The differences were not statistically significant. Thirty-six patients in group 1 had a malignant renal mass; 21 were a result of kidney metastasis and the remaining 15 were a synchronous primary renal tumor (0.9% vs 0.6%). The kidney was the eighth most common site of metastatic spread. Metastases to the kidney were statistically more likely with higher tumor stage of the primary nonrenal malignancy (68% vs 46%, p = 0.0006) and in those with other sites of metastasis (p = 0.012, positive likelihood ratio [LR+] = 6.75). Compared with primary renal tumors, metastases to the kidney were more often solid (86% vs 53%, p = 0.019, LR+ = 3.7) and endophytic (76% vs 33%, p = 0.017, LR+ = 2.29). There were too few cases with calcification and bilateral tumors to reach a statistically significant conclusion. Tumor size, polar predominance, and enhancement pattern were similar in the two groups. The primary renal tumors seen in group 1 versus group 2 were similar regarding age and sex distribution, cell type, median size, and tumor stage.Metastases to the kidney are uncommon in modern radiologic practice (0.9%, 21/2340 in this study), and a renal mass seen in a patient with nonrenal malignancy is nearly as likely to be an incidental primary renal tumor. Metastasis is more likely in those with higher tumor stage or if other viscera are also affected and is usually an asymptomatic, small, endophytic, and solid mass. If a renal mass seen in a patient with primary nonrenal malignancy proves to be a synchronous primary renal tumor, its cell type and stage will be similar to sporadic primary renal tumors.