The mean annual rate of increase in the length of pagetic lesions was 8.5 mm for the skull and tibia and 9.4 mm for the femur, after a follow-up of nine to 16 years according to the bone. The fastest rate of progression was seen at the femur and was 24 mm per year. Thirty years were required for lesions to spread to the entire pelvis and 13 years to all the bones surrounding the obturator foramen. Saber shin deformity of the tibia without involvement of the distal fourth of the bone indicated a disease duration of 25 years, as did involvement of the entire skull. The annual rate of increase in the width of lesions varied widely across patients and was not influenced by gender. Thickening of the skull occurred at a rate of about 4 to 5 mm per decade after pagetization of the bone, although faster rates were seen in some patients; a sandwich-like appearance with a thickness exceeding 32 mm was seen in six of the 29 skulls studied. At the femur and tibia, the increase in width was 10% to 30% per decade after pagetization of the bone; faster thickening was seen in some tibias with saber shin deformity. The thickness of the ischial tuberosity increased by 3 to 4 mm per decade after pagetization. Determination of the degree of hypertrophy is useful for estimating the duration of pagetic lesions when the entire bone is involved at first presentation. Involvement of the entire pelvis indicates a disease duration of 30 to 40 years according to whether the bone is hypertrophied or not. An estimation of age at disease onset in 70 patients suggested that the first bone lesions probably appeared before the age of 30 years in 45 patients (64%), whereas the diagnosis was established before 30 years in only three patients. These data suggest that Paget's disease may be a disease of teenagers and young adults.