This study compares the biological performance of three fluidised-bed biological reactors under conditions of different headspace pressures. The application of pressure can have a profound effect on the initial rate of bed growth. However, once the fluidised-bed reaches full expansion, the biological performance at higher pressures is greater than those at lower pressures. There appears to be an almost linear relationship between the application of pressure and the performance of the fluidised-bed biological reactors in removing soluble BOD5. This can be attributed to the increase in the oxygen concentration in the bulk liquid and a greater oxygen penetration depth within the biofilm.