Eighteen of the patients randomized for treatment according to the first and second National Wilms' Tumor Studies have developed clinically evident tumor in the contralateral kidney subsequent to the initial diagnosis. Only seven of 18 had an adequate exploration of the opposite kidney during the initial surgery and of these seven, four had abnormal findings. Thirteen of 18 cases showed some form of nephroblastomatosis histologically. The original tumors in six and possibly seven were multicentric in origin. While metachronous Wilms' tumors are an unusual occurrence, the prognosis is ominous with only seven of 18 surviving free of disease. The outlook might be improved with adequate attention to the surgical and histologic details during the initial diagnostic procedures.