This study was conducted to improve hydrogen fermentation of food waste in a leaching-bed reactor by heat-shocked anaerobic sludge, and also to investigate the effect of dilution rate (D) on the production of hydrogen and metabolites in hydrogen fermentation. Among various reaction constraints affecting the fermentation of food waste, a key factor is the adjustment of environmental conditions during the fermentation because various components of food waste have different characteristics of degradation. D was used as a tool to keep the optimum conditions of hydrogen fermentation. The fermentation efficiency (58%) at initial D of 4.5 d(-1) was higher than those (51.4, 55.2, and 53.7%) at initial D of 2.1, 3.6, and 5.5 d(-1). The chemical oxygen demand (COD) removed was converted to hydrogen (10.1%), volatile fatty acids (VFA) (30.9%), and ethanol (17.0%). The butyrate/acetate (B/A) ratios were maintained over 3.2 in the first 2 days. In addition, the fermentation efficiency improved from 58.0% to 70.8% by adjusting D from 4.5 to 2.3 d(-1) depending on the state of degradation. The COD removed was converted to hydrogen (19.3%), VFA (36.5%), and ethanol (15.0%). Compared to 0.7-2.2 with no D control, the B/A ratios were kept high (2.0-2.7) on days 3-7, accompanied by the second hydrogen peak. The trend of B/A ratios was similar to the hydrogen production. D control led environmental conditions to favor hydrogen production. This meant that the fermentation efficiency was improved by the enhanced degradation of slowly degradable matters. Moreover, D control could delay the shift of predominant metabolic flow from hydrogen- and acid-forming pathway to solvent-forming pathway. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.