Ultrasound is an ideal imaging modality that may be used to evaluate scrotal masses. It will frequently detect the presence of a mass, localize its site of origin, and characterize the abnormality. The most valuable information provided by ultrasound is the separation of intra- and extratesticular lesions. The importance of this distinction is understood when it is recognized that the majority of intratesticular lesions are malignant, while more extratesticular lesions are benign. The accuracy in this determination is between 90% and 95%. The overall accuracy in detecting and characterizing soft tissue masses as cystic or solid is in the range of 90% to 100%. Once a solid mass is identified, it is not possible to distinguish benign from malignant. We present the ultrasound findings of a newborn with renal vein thrombosis and a left scrotal enlargement, and discuss its pathogenesis.