Dysfunction of spinal stability seems to be one of the causes of low back pain (LBP). It is thought that a large number of muscles have a role in spinal stability including the pelvic floor muscle (PFM). The purpose of this study was to investigate the effect of PFM exercise in the treatment of chronic LBP. After ethical approval, a randomized controlled clinical trial was carried out on 20 women with chronic LBP. Patients were randomly allocated into two groups: an experimental and a control group. The control group was given routine treatment including electrotherapy and general exercises; and the experimental group received routine treatment and additional PFM exercise. Pain intensity, functional disability and PFM strength and endurance were measured before, immediately after intervention and at 3 months follow-up. In both groups pain and functional disability were significantly reduced following treatment (p<0.01), but no significant difference was found between the two groups (p>0.05). All measurements were improved in both groups (p<0.01) although patients in the experimental group showed greater improvement in PFM strength and endurance (p<0.01). It seems that the PFM exercise combined with routine treatment was not superior to routine treatment alone in patients with chronic LBP.