The relationship of low back pain (LBP) to various socio-economic factors and previous sickness absence was studied in a random sample of 940, 40-47 year old, men. Since most of the data were available from official registers a comparison between participants and non-participants was possible. The participation rate was 76%, and the life-time incidence of LBP was 61%. No differences were found between men with and men without LBP with respect to education, changes in income, and family conditions. Among the men with LBP there was a tendency to a higher proportion of foreign citizens. The LBP men had an increased previous sickness absence and on average a lower sickness benefit. A higher proportion of the LBP men were blue collar workers. When the influence of other variables was assessed by an analysis of co-variance, two of the variables maintained a direct association with LBP, viz. foreign citizenship and an increased number of sickness absence days. A lower average sickness benefit, an increased previous sickness absence, and a larger proportion of unmarried and divorced men were found among the non-participants who were also more frequently foreign citizens and blue collar workers.