Correct interpretation of measurements of water consumption in buildings over a short period of time, e.g. one or two weeks, requires knowledge of the seasonal variation of this consumption. In this study the seasonal variation of water consumption in 8.046 apartments of in total 390 apartment buildings in Belgium has been analysed. The data available consist in monthly consumption records of hot and cold water for each apartment, and this over a period of 3 years for each apartment, between 2008 and 2012 for all buildings. In the domestic hot water (DHW) consumption a seasonal variation was found : in winter DHW consumption was found to be about 12% higher than the average consumption, whilst in summer this consumption was almost 13% lower than average. This seasonal variation affects as well the mean daily consumption as the peak consumption on a short time base, i.e.10 minutes. The influence of the seasonal variation of the cold water temperature on the hot water consumption was identified as an important influencing factor.