Long-term effect of hydroxyurea therapy on TCD flow velocities. Children with SCA and increased baseline TCD velocities received hydroxyurea therapy at MTD. The maximum TCD velocity is shown after reaching MTD and then in long-term follow-up for 28 children. There was a significant decrease from baseline TCD flow velocities to the MTD values (P < .001), and this decrease was sustained at long-term follow-up.