Inpatient and outpatient treatments were compared with a control intervention in 288 men and 168 women, aged 35-54, who were at work, but suffered from chronic or recurrent low back pain. Physical measurements and back pain assessments were carried out before the intervention and at a 3-month follow-up. Physical fitness improved most in the inpatients, but the outpatients did not differ from the controls. Correlations between back pain and physical measurements indicated that increase of lumbar and hip mobility was more important than increase of trunk strength for subjective progress in these patients. Increased trunk extension strength correlated significantly with subjective progress in women, who also had higher correlations between improved physical fitness and progress than men.