The development of a new questionnaire to measure social support, the Close Persons Questionnaire is described from the Whitehall II Study, a longitudinal study of the impact of psychosocial factors on health. The Close Persons Questionnaire includes items on three types of support from up to four nominated close persons and social networks. Its repeatability over a four week period and its validity against the Self Evaluation and Social Support Interview is reported in a sample of British Civil Servants. The difficulties of satisfactorily validating measures of social support are discussed.