As regulations governing the disposal of clinical waste become more stringent, the use of incineration is increasing. Every year in the UK more than 0.7 Mt of hospital waste is generated; most of it is infectious, and must be incinerated. This paper reports on the performance of a newly commissioned high-temperature incinerator plant with a burning capacity of 12 t/day (4 MW); the technology used is outlined, and a brief description of the emission-monitoring programme is presented. The testing included the measurement of CO, CO2, O-2 and hydrogen chloride (HCI) concentrations in the flue gases, total particulates and dioxins/furans. Scrubber liquor, filter cake and ash were also analysed for heavy metals and unburnt carbon content. A mathematical model of the finite-difference type (FLUENT) was then employed to predict the three-dimensional reacting flows (gaseous phase) within the incinerator geometry. The main objective of this modelling work was to investigate the influence of the design and operating parameters on the overall performance of the incinerator. Experimental measurements of gas composition, temperature and velocities were compared with the model predictions. Modelling results indicated trends correctly, and were invaluable for the interpretation of the incinerator performance data. As a result of the test data and mathematical modelling of the whole process, suggestions for design improvements were made. Three secondary-air injection systems were investigated by means of the CFD model, in an attempt to obtain optimum combustion conditions that would minimise potential emissions of toxic pollutants and extend the mean gas residence times. The modelling work showed that the use of six high-speed secondary air jets firing towards a common centre in the secondary combustion chamber (after-burner) produced substantially longer residence times, improved the temperature profile at the exit, reduced the concentration of toxic pollutants, and raised the combustion efficiency of the plant.