From November 1990 to March 1993, seven children with 10 congenitally hyperextended knees were investigated by ultrasound (US). Clinical, radiographic, and US examinations were started as early as the second day after birth. Sonographic controls were continued through the first year of life. Leveuf's classification of the deformity (19) could be confirmed. Three knees had true dislocation, five had subluxation, and two had simple hyperextension. US with a 7.5-MHz linear transducer with a ventral or lateral approach can replace radiographic or arthrographic documentation, avoiding the disadvantages of ionizing radiation. Success or failure of conservative treatment can be followed on repeated US images, allowing optimal timing for the change from conservative to surgical management of deformity.