A family tree of unilateral hydronephrosis is presented. The disease occurs in three consecutive generations. Although the size of the family and the number studied are too small to draw conclusions as to the hereditary pattern or type of transmission, it is noted that all positive members are males and that the characteristic is transmitted through the males. Heredity plays a part in the etiology of unilateral hydronephrosis. The abnormality may be present at birth in a marked degree. Symptoms and decrease in kidney function in the affected kidney are noted in the patients in advanced years.