A long-term follow-up study was made of 23 patients with an average age of 47 years (range 38-68 years) who had adolescent Blount's disease. Four patients had bilateral disease (27 affected knees). Nine knees had no treatment during childhood, 11 were treated by physiodesis, and seven were treated by osteotomy of the proximal tibia. At follow-up, most of the patients had no pain or mild pain from their knee. Nine knees showed arthrosis. We conclude that most children with adolescent Blount's disease will have a straight leg at middle age without arthrosis and that this result can be obtained in one of four patients without treatment.