The prevailing local climatic conditions affect the building loads and the heating, ventilating and air conditioning equipment sizing and performance. This paper presents work on the determination of outdoor design conditions for cooling load calculation. The computer package DOE-2.1E was used to simulate the hourly cooling load of a generic commercial office building in Hong Kong. Totally, 22 weather data sets, from 1979 to 2000, were used for the analysis. The findings indicate that the outdoor climatic conditions developed for cooling load estimation via the simulations are less stringent than the current outdoor design data and approaches adopted by local architectural and engineering practices. Building designers should select and evaluate critically the outdoor design conditions in order to achieve optimum air conditioning equipment sizing.