A semi-active control device for structural control implementations is presented and discussed in this paper. Based on two passive control devices, the mass pump and the hydraulic mass system, a new passive control system, the mass damper pump (MDP), is introduced. It is found that the MDP system is more effective in vibration control than the other two passive control systems. It is then shown that the passive control MDP can be modified to be a semi-active control device and is very effective in structural control. Through theoretical analysis and numerical simulations, it is found that the proposed semi-active control device MDP is more suitable for structures with ordinary height or the magnitude of earthquake excitations is not very large (because the control force provided by the semi-active control is limited). Under these situations, the maximum response of a controlled structure can be reduced by one-third to one-half. Also, it is found that multiple MDP is more effective in reducing structural response than a single MDP when they are placed in appropriate locations. Copyright © 2005 John Wiley & Sons, Ltd.