It is generally recognized that social relationships occupy a central position in psychiatry. To examine the role of social relationships in the onset of minor psychiatric morbidity it is necessary to construct an instrument which will meet the need for a valid, reliable and comprehensive index of social relationships. Such an instrument has now been developed and used in a sample of the general population. The Interview Schedule for Social Interaction arose from a research need, and was constructed to assess the availability and perceived adequacy for any individual of a number of facets of social relationships. These consist both of persons and of the provisions obtained through them. Data from a general population sample suggest this instrument to be sufficiently valid and reliable, and also sensitive to predictable variations between sociodemographic groups, to justify its use in clinical and epidemiological studies, both in psychiatry and general medicine.