A case of nephroblastomatosis is described in a previously healthy child with a palpable mass in the right upper quadrant as an incidental finding. This case of nephroblastomatosis demonstrates the whole spectrum of abnormalities from local regression to formation of true Wilms' tumors bilaterally over a 4-year period. The evolution of these changes provides a graphic and dramatic example of this interesting entity, indicating its potential malignant association with Wilms' tumor.