This paper presents the real-time sensing-based risk-mitigation control of hydrogen dispersion and accumulation in a partially open space with low-height openings by forced ventilation. In the partially open space we previously considered (Matsuura et al., Int J Hydrogen Energy, 35(10), p. 4776-4786 (2010)), a hydrogen buoyant plume is subjected to cross flows during forced ventilation, and hydrogen travels over a long distance in the lower part of the space, which enhances the hydrogen concentration there. On the basis of those results, we alternatively propose in this paper a new partially open space that permits the almost vertical rising of hydrogen from a leak source, and exhausts, by forced ventilation, hydrogen temporarily accumulated near the roof. We first describe computational geometries and scenarios, mathematical models and numerical procedures. Related to this model, an acceptability diagram of exhaust flow rates for various leak flow rates and leak positions, an estimation method of the instantaneous hydrogen amount accumulating near the roof, and a hunting-preventive control scheme of the exhaust flow rate based on the least-squares method are constructed. A sensing-based risk mitigation control strategy to change the exhaust volume flow rate is then proposed. The effects of the number of sensors on the proposed control system are also investigated. The proposed system is validated for various leak positions and leak modes. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.