Back pain (BP) has been rated among the most important factors affecting physical health status in old age. Yet there is an under-representation of the older population in the BP literature. We present extensive interview data from the Longitudinal Study of Aging Danish Twins, dealing with a population-based sample of Danish twins aged 70-102, and describing the 1-month prevalence of BP and the development of BP over time. The associations between BP and education, self-rated health, other health problems, lifestyle factors, and physical and mental function were also investigated. Data were analysed in a cross-sectional analysis for all answers given at entry into the study and in a longitudinal analysis for participants in all four surveys. Associated factors were analysed for the cross-sectional sample using univariate and multivariate analysis accounting for the non-independence of twins in complete pairs. The overall 1-month prevalence of BP was 25% and differed significantly between men and women. The variations in prevalence between the age groups and over time were negligible. The majority of participants in all four surveys had either not experienced BP during the previous month or had done so on one occasion only. Education was not associated with BP. Self-rated health was associated with BP in a significant "dose-response" like pattern. BP was associated with bone and joint disorders, migraine headaches, lung disease, cardiovascular disorders and gastric ulcer, but not neurologic or endocrinologic diseases. BP sufferers had significantly lower scores on physical but not on mental functioning. We conclude that BP is a common symptom in old age; however, the prevalence does not change with increasing age. BP may be part of a more general syndrome of poor health among the old.