Abstract Background Telephone triage is a core but vulnerable part of the care process at out-of-hours general practitioner (GP) cooperatives. In the Netherlands, different instruments have been used for assessing the quality of telephone triage. These instruments focussed mainly on communicational aspects, and less on the medical quality of triage decisions. Our aim was to develop and test a minimum set of items to assess the quality of telephone triage. Methods A national survey among all GP cooperatives in the Netherlands was performed to examine the most important aspects of telephone triage. Next, corresponding items from existing instruments were searched on these topics. Subsequently, an expert panel judged these items on importance, completeness and formulation. The concept KERNset consisted of 24 items about the telephone conversation: 13 medical, ten communicational and one regarding both types. It was pilot tested on measurement characteristics, reliability, validity and variation between triagists. In this pilot study, 114 anonymous calls from four GP cooperatives spread across the Netherlands were judged by three out of eight raters, both internal and external raters. Results Cronbach’s alpha was .94 for the medical items and .75 for the communicational items. Inter-rater reliability: complete agreement between the external raters was 45% and reasonable agreement 73% (difference of maximally one point on the five-point scale). Intra-rater reliability: complete agreement within raters was 55% and reasonable agreement 84%. There were hardly any differences between internal and external raters, but there were differences in strictness between individual raters. The construct validity was confirmed by the high correlation between the general impression of the call and the items of the KERNset. Of the differences within items 19% could be explained by differences between triage nurses, which means the KERNset is able to demonstrate differences between triage nurses. Conclusions The KERNset can be used to assess the quality of telephone triage. The validity is good and differences between calls and between triage nurses can be measured. A more intensive training for raters could improve the reliability.