Extrarenal Wilms' tumor is a very rare disease, and usually occurs in pediatric patients. We present a case of extrarenal retroperitoneal Wilms' tumor in a six-year old girl with a six-month history of a palpable left abdominal mass. The ultrasonographic and CT features of this tumor showed a well-defined, large, inhomogeneous, predominantly solid mass which was separate from the left kidney. Surgical pathology confirmed this to be an extrarenal Wilms' tumor.