The characteristics of flow distribution in a complex duct system have been investigated in this paper by three means: the experimental measurement, the numerical simulation and the extended T-method analysis. While the exit flow rates predicted by the three-dimensional CFD calculation and those given by the experiment show close agreement, the extended T-method results are found to differ from the experiment by - 22.2 to 26.3 per cent for the same exits. These discrepancies may be attributed to the underlying limitation concerning the fitting loss coefficients, which is that the flow in front of the fittings is fully developed. It is proposed that further improvements to the fitting loss coefficients should be made in order to analyse the flow distributions in complex duct systems by one-dimensional analysis such as the extended T-method. Copyright (C) 2001 John Wiley & Sons, Ltd.