Symptoms of intervertebral foraminal stenosis are caused by compression of nerve root exiting the intervertebral foramen. Many attempts to measure the size of the neuromuscular exit have been made; however, only a few studies to compare the area differences between foramens by computed tomography (CT) were done. In this retrospective comparative study, we used the region of interest (ROI) in CT to measure and compare the area of intervertebral foramen between the healthy control group and the patient group.Eighty-one patients who underwent CT of the lumbar spine between May 2014 and December 2017 were enrolled. Using the medical imaging program, the foraminal area between L5 and S1 vertebrae was measured on the sagittal, coronal, and axial planes using ROI. Four groups were established for comparison: those diagnosed with foraminal stenosis by a radiologist and those who were not, those diagnosed with foraminal stenosis by orthopedic surgeons and those who were not. These groups were further divided into subcategories depending on whether the area was operated on for foraminal stenosis. Interobserver and intraobserver agreements were assessed.The mean age of patients was 56.5 years (range 17-84). The foraminal area of the surgical group on sagittal plane was significantly narrower than the control group (P = .005). However, the difference between the 2 groups on axial and coronal planes was not statistically significant (P > .1). Foraminal area <80 mm on sagittal images was a statistically significant risk factor for clinical symptom (P = .028) and that <65 mm was a statistically significant risk factor in predicting operability (P = .01). Interobserver and intraobserver agreements were fair to good on axial and coronal planes (about 0.7), whereas the agreements were excellent on sagittal plane (>0.9).In this study, we proved that measuring the intervertebral foraminal area using the ROI in CT in the lumbar spine is useful for diagnosing L5-S1 foraminal stenosis, especially on sagittal plane. Furthermore, not only does it provide aid in diagnosis, but it also helps predicting the operability of foraminal stenosis.