A randomised sample of 940, 40 to 47 year old, men were asked to participate in an investigation where low back pain (LBP) was studied. 716 men (76%) were personally investigated and information concerning the remaining men was received from the Swedish National Health Insurance Office. The life time incidence of LBP was 61%, the prevalence 31%. 40% of the participants with LBP had sciatica. The disability prevented work in 3.6% of the participants and 4% had been off work more than three months because of LBP in the three years preceding the study (1975-1977). 40% had consulted a physician, 3.5% had been admitted to a hospital, and 0.8% had been operated on because of their LBP. Men with LBP had significant functional impairment as reflected by e.g. decreased ability to flex their spine, to lift and to perform activities of daily living and sports. According to National Health Insurance data about previous sickness 49% of the participants and 59% of the non-participants had at some time been off work because of LBP. Among the participants who, during the interview, stated that they had never had LBP there were 27% who had been sicklisted with an LBP diagnosis. Thus the incidence and prevalence rates were underestimated when based on interview findings from the participants.