The influence of urban heat advection on the temperature time series of De Bilt (1897–present) has been studied by comparing the hourly meteorological observations (1991–2000) with those of the nearby (7 km) rural station Soesterberg. The measuring site at KNMI is surrounded by three towns: De Bilt (33,000 inhabitants) extending from KNMI to the north, Utrecht (234,000 inhabitants) at 2.5 km west and Zeist (60,000 inhabitants) at 2.4 km southeast. The hourly temperature differences between De Bilt and Soesterberg have been examined with respect to wind direction and cloudiness. Strong wind direction dependence was apparent for clear days, with the greatest differences for wind coming from the cities (up to 1 °C on average). Both the magnitude and wind direction dependence of the temperature differences decreased with increasing cloudiness. On average, the temperature difference is of the order of tenths of degrees Celsius, which is small but important with respect to the climate signal in long-term temperature records like that of De Bilt. The De Bilt time series has been used to monitor climate change in the Netherlands and is comprised in the world databases of land-surface air temperature. It is also part of the longer compound time series called the Zwanenburg/De Bilt time series (1706–present). This latter series is one of the scarce very long instrumental time series in the world. A study of the influence of urbanization enhances the understanding of the long-term trend and variations in the temperature series.