Eight hundred and two subjects employed in N.W. England have been interviewed and examined after episodes of back or sciatic pain, using a standardized procedure. Data on recurrence of symptoms leading to further treatment or absence from work in the following two 12-month periods were obtained by postal questionary. Residual pain in the leg and a number of positive clinical signs of return to work, longer sickness--absence for the current attack, and two or more previous attacks were all associated with recurrence or persistence of symptoms. The prognosis also varied according to the cause of back pain, falls being associated not only with longer periods of absence in the current attack but with a higher rate of recurrence. The results have underlined the significance of a thorough examination on return to work after back pain for the industrial medical officer, as well as the epidemiological importance of this phase in the natural history of back and sciatic pain.