Parents of children who weigh more than 18 kilograms (about 40 pounds) may be asked if extra blood may be drawn after 3 months (visit 3) and 6 months (visit 4) of treatment to measure letrozole levels. The blood will be drawn before the morning dose and at 0.5, 1, 1.5, 2, 3, 4, 6, 8, and 24 hours after the dose through an indwelling needle placed in the vein for 8 to 24 hours.