In this article account is given of a validation study, according to the nursing diagnosis 'social isolation'. The label, definition and cluster of defining characteristics according to cluster of defining characteristics according to Townsend (1990) are the starting point, the related factors are left out of consideration. The validation mentioned above was carried out in accordance with the Delphi-method, that consisted of three Delphi rounds. A panel of experts was asked particularly to compile defining characteristics that are relevant for 'social isolation' and to indicate the critical defining characteristics. This procedure generated a cluster of seven defining characteristics for 'social isolation' of which the content validity turned out to be considerably increased with regard to the cluster formulated by Townsend. The distinction between critical defining characteristics and supporting defining characteristics was not clarified by this study.