The purpose of the study was to determine the relationships among pain, trunk extension and trunk flexion muscle strength and functional status in patients suffering from low back pain (LBP). 20 female patients suffering from LBP were included in this study. The patients (average age of 59.6 years) were treated with manual therapy using the Kaltenborn-Evjenth technique in conjunction with a physical therapy program. The treatment consisted of 5 sessions per week for a total of 2 weeks. Back pain, trunk extensor and flexor strength and functional level were evaluated before and after the treatment. The patients showed significant improvements in pain, muscle strength and functional level after the treatment (p < 0.05). Greater muscle strength was significantly correlated with higher functional level (r = -0.596 for trunk flexors, r = -0.547 for trunk extensors). There was an inverse correlation between pain intensity level and functional level (p < 0.05, r = 0.730), while there was no correlation between back pain and strength of the trunk muscles. We concluded that increased trunk muscle strength induces improvement of functional level which, in turn, reduces pain.