A comparative study between Wilms' tumors and neuroblastomas, which are the most common intraabdominal massesin pediatric age, was performed with emphasis on the ultrasonographic findings. Seventeen cases of neuroblastomasand fifteen cases of Wilms' tumors proven histologically were analyzed in their ultrasonographic characteristicsand with special attention to the ecogenicity, sharpness of the margin, and the presence of necrosis orcalcification there-in. The neuroblastomas(17 cases) were usually heterogeneous(15 cases, 85%) and hyperechoic(8cases, 47%) with regard to the echogenicity of liver, while Wilm's tunmors(15 cases) were homogenous(13 cases,87%) and isoechoic (14 cases, 93%). The margin of the mass was rather ill-defined (9 cases, 53%) in neuroblastoma,but well defined in wilms' tumor (14 cases, 93%). The incidence of the tumor necrosis of both groups was similar(47%, 40%, respectively), but the degree of necrosis was variable. The neuroblastomas had calcification verycommonly (14 cases, 82%), but the wilms' tumor had only few(1 case, 7%).