Study Design. Data were obtained in a Danish cross-sectional postal survey and compared with information from four methodologically similar studies conducted in some of the Nordic countries between 1977-1985. Objectives. The objectives were to estimate the lifetime cumulative incidence and the 1-year period prevalence of low back pain in the general population, to study whether there are any differences in the occurrence of low back pain according to age and sex, and to investigate whether low back pain is on the increase. Summary of Background Data. The prevalence of low back pain commonly is thought to be high, but estimates differ considerably between studies. It is also not known whether low back pain is more common in men or women or in certain age groups, and it is unclear whether the prevalence of low back pain has increased in the past years. Methods. Prevalence estimates were established in a current study, and results then were adjusted to suit the age and sex criteria of four previous studies. Results. Between 60-65% of 30- to 50-year-old men and women living in the Nordic countries reported at least one incident of low back pain during their lifetime, based on the information from four studies with a total sample size of 3513. The most likely 1-year period prevalence estimate is between 44-54%, based on two studies and a total sample of 2035 individuals. There was no consistent evidence favoring higher figures with increasing age or relating to any of the genders. No clearly observed time-related trend was noted. Conclusions. When data were examined from five methodologically similar studies on the 30- to 50-year-old Nordic population, there was reasonable consistency of prevalence figures. Thus, approximately 66% report having had low back pain at least sometime during their lifetime and approximately 50% sometime during the preceding year, with no significant differences relating to age or sex. The best method to investigate whether low back pain is on the increase might be through replicate studies.