The effect of influent COD/N ratio on biological nitrogen removal (BNR) from high-strength ammonium industrial wastewater was investigated. Experiments were conducted in a modified Ludzack-Ettinger pilot-plant configuration for 365 days. Total nitrification of an influent concentration of 1200mg NH4+-Nl(-1) was obtained in this period. Influent COD/N ratios between 0.71 and 3.4g CODgN(-1) were tested by varying the nitrogen loading rate (NLR) supplied to the pilot plant. An exponential decrease of nitrification rate was observed when the influent COD/N ratio increased. The experimental COD/N ratio for denitrification was 7.1 +/- 0.8 g COD g N-1 while the stoichiometric ratio was 4.2 g COD g N-1. This difference is attributable to the oxidation of organic matter in the anoxic reactor with the oxygen of the internal recycle. The influence of influent COD/N ratio on the treatment of high-strength ammonium industrial wastewater can be quantified with these results. The influence of COD/N ratio should be one of the main parameters in the design of biological nitrogen removal processes in industrial wastewater treatment. (C) 2003 Elsevier Ltd. All rights reserved.