In order to justify the selection of therapeutic measures for low-back pain and sciatica, epidemiologic and clinical studies were performed of the natural histories or the long-term prognosis of those symptoms. The main conclusions drawn in relation to decision making are as follows: 1) low-back pain is more frequent than sciatica or intermittent claudication, but the latter is more disabling; 2) acute attacks are generally more disabling than chronic pain, and the frequency may be more closely related to poor prognosis than the duration; 3) radiologic findings are of little value in differentiating the incidence and degree of the symptoms during life; 4) myelographic or peridurographic abnormalities do not always suggest poor prognosis.