Ambulance services are in operation around the world and yet, until recently, ambulance data has only been used for operational purposes rather than for assessing public health. Ambulance call-out data offers new and valuable (near) real-time information that can be used to assess the impact of environmental conditions, such as temperature, upon human health. A detailed analysis of London ambulance data at a selection of dates between 2003 and 2015 is presented and compared to London temperature data. In London, the speed of ambulance response begins to suffer when the mean daily air temperature drops below 2 °C or rises above 20 °C. This is explained largely by the increased number of calls past these threshold temperatures. The baseline relationships established in this work will inform the prediction of likely changes in ambulance demand (and illness types) that may be caused by seasonal temperature changes and the increased frequency and intensity of extreme/severe weather events, exacerbated by climate change, in the future.