Hydrogen dispersion and accumulation in a partially open space with natural and forced ventilation are computationally investigated in this paper. After validating the present numerical method and assessing grid width against experimental data of a hallway, the present method is applied to a garage with dimensions of 2.6 m x 2.6 m x 2.6 m and leak flow rates of 0.5-4 standard cubic feet per minute (SCFM). In the simulations, different combinations of roof vents and door vents are considered by changing their sizes and positions. Based on the results, differences in concentration increase, the processes of hydrogen dispersion and accumulation, unsteadiness and vortical structures accompanying ventilation, are shown for natural and forced ventilation. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.