The authors present the experience acquired by the Urology Department of "La Paz" Hospital in the sonographic diagnosis of hydronephrosis over a period of one year. In 47 patients, the percentage of successful diagnoses reached 93.7%, including 32 cases with a silent kidney in the intravenous urography. They discuss the different sonographic patterns of hydronephrosis, in relation to its degree of evolutionary development and its differential diagnosis with other kidney disorders.