Techniques for assessing daily function in Patients with back pain are generally crude and limited in scope. We therefore examined a "health status" questionnaire, the Sickness Impact Profile (SIP) to assess its measurement characteristics in such patients. Eighty patients with mechanical low-back pain completed the SIP and a physical examination at a walk-in visit and again three weeks later. Test-retest reliability of the SIP was substantial. Biologic validity was confirmed by significant correlations with age, pain duration, spine flexion, straight leg raising, and pain severity. Validity of psychosocial subscales was confirmed by significant associations with patient anxiety and psychiatric problems. Scores changed in the expected directions when patients were evaluated three weeks later. The SIP is thus valid, reliable, sensitive to clinical changes, and comprehensively assesses a wide range of dysfunctions.