The correlation between lumbosacral transitional vertebrae and low back pain was analyzed in a study of plain radiographs from normal subjects and patients with chronic low back pain. Of 184 normal subjects, only 29 (15.8%) had transitional vertebrae, while 97 (35.1%) of 276 patients were noted to have transitional vertebrae. The difference was highly significant (p < 0.01). The incidence of Castellvi type II transitional vertebrae was significantly higher in patients with low back pain than in controls (p < 0.01). This study indicates the etiologic significance of lumbosacral transitional vertebra in low back pain.