We present the development and standardization of a measure of spiritual, religious and philosophical beliefs. An interview was constructed based on on-going studies by the authors of the nature and strength of belief held by people hospitalized with an acute illness. The interview was tested with three standard populations--staff of a teaching hospital; attenders to an inner city general practice; and people with clearly defined, devout religious beliefs--in order to establish population norms, validity and reliability for each question. The interview performed well with satisfactory validity and high internal and test-retest reliability. It is not presented, however, as a final product which will meet all needs in this complicated area of study. Rather, we have attempted to refine a measure of spiritual and religious belief that might apply to people with a range of personal and public faiths. It is clear that people are able to express these aspects of their lives in a way that can be measured with acceptable reliability and validity. We believe that this interview could, therefore, be applied in any medical, psychological or social setting in which a measure of belief is sought.