[Purpose] It has been reported that chronic low-back pain (CLBP) patients exhibit asymmetric atrophy of the lumbar multiidus muscle. However, studies focusing on the abdominal muscles have not yet been conducted. The purpose of this study was to determine abdominal muscle thickness and symmetry in CLBP patients and healthy subjects. [Subjects and Methods] Data were obtained from 50 healthy subjects (30.2 ± 6.1 years) and 50 CLBP patients (31.5 ± 8.7 years). The thicknesses of the rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) were measured by ultrasonography. We calculated the mean thicknesses of both sides. In addition, we calculated the asymmetry ratio as a percentage of the difference between the sides. The differences in muscle thickness between the CLBP patients and healthy subjects were analyzed using the t-test. [Results] We found that the TrA thickness was signiicantly smaller in CLBP patients (3.7 ± 0.8 mm) than in healthy subjects (4.2 ± 1.2 mm). Similarly, the TrA asymmetry ratio was signiicantly different between the healthy subjects (8.4 ± 7.5%) and CLBP patients (16.4 ± 12.1%). [Conclusion] The TrA muscle of CLBP patients was thinner and more asymmetric than that of healthy subjects.