The prevalence of low back pain (LBP) has been reported in the literature for different populations. Methodological differences among studies and lack of methodological rigor have made it difficult to draw conclusions from these studies. A systematic review was done for adult community prevalence studies of LBP published from 1981 to 1998. The technique of capture-recapture was performed to estimate the completeness of the search strategy used. Established guidelines and a methodological scoring system were used to critically appraise the studies. Thirteen studies were deemed methodologically acceptable. Differences in the duration of LBP used in the studies appeared to affect the prevalence rates reported and explain much of the variation seen. It was estimated that the point prevalence rate in North America is 5.6%. Further studies using superior methods are needed, however, before this estimate can be used with confidence to make health care policies and decisions relating to physical therapy.