The Guideline on Odour in Ambient Air has been in use for many years for odour regulation in Germany. The main parameter that the odour regulation authority has to take into account is the odour frequency expressed as odour hours per year. In the guideline, limit values are given for the maximum odour frequency per year. These limit values are based on field investigations in which significant relationships between odour impact and odour annoyance was found. In these investigations, odour intensity did not yield a better description of the degree of annoyance caused to the residents. The hedonic tone was not mentioned. In a new research project finished in 2003, the influence of odour intensity and hedonic tone, in addition to odour frequency, has been assessed. Two installations emitting pleasant odours, two emitting neutral and two emitting unpleasant odours, have been selected. In each case grid field measurements by a panel were carried out, and the annoyance of the residents was assessed using a special questionnaire. The results of this project are: (1) A new method to measure odour intensity and hedonic tone in the field with data record forms was developed and validated. With this method, reliable and reproducible results are obtained. (2) The parameter odour frequency based on the system of "odour hours" is suitable and sufficient to predict the odour annoyance caused by unpleasant/neutral odours. (3) In the case of pleasant odours, hedonic tone has an abundantly clear effect on the dose-response relationship between odour frequency and annoyance. Pleasant odours have a significant lower annoyance potential than unpleasant/neutral odours. (4) The odour intensity has no additional influence on this relationship. If odours are recognisable then they can cause annoyance.