A quantitative study of the trunk muscle strength and fatigability was performed using an isokinetic dynamometer to determine the role of trunk muscles in the low-back-pain syndrome and in particular to investigate the correlation of trunk muscle strength and lumbar lordosis. Most patients demonstrated a generalized weakness of the trunk muscles. The trunk muscle strength of patients suffering from backache for less than one month was significantly lower than that of the controls. The strength of the trunk muscles was not influenced by the duration of symptoms. There was no imbalance between the trunk flexors and extensors. The patients with back pain had a greater fatigability of the trunk flexors than the controls. There was negative correlation between the trunk muscle strength or fatigue and lumbar lordosis.