This report describes the results of research conducted on the reliability of the Sickness Impact Profile (SIP). The SIP is a questionnaire instrument designed to measure sickness-related behavioral dysfunction and is being developed for use as an outcome measure in the evaluation of health care. The test-retest reliability of the SIP in terms of several reliability measures was investigated using different interviewers, forms, administration procedures, and a variety of subjects who differed in terms of type and severity of dysfunction. The results provided evidence for the feasibility of collecting reliable data using the SIP under these various conditions. In addition, subject variability in relation to reliability is discussed.