This study was undertaken to establish normal reference ranges for abdominal muscle size and symmetry and to examine the effects on these of gender and age. We studied 123 subjects, consisting of 55 men (aged 21-72 years) and 68 women (aged 20-64 years). Real-time ultrasound imaging of the abdominal muscles was performed. Thickness of internal and external oblique (IO, EO), transversus abdominis (TA), and rectus abdominis (RA), and cross-sectional area (CSA) of RA were measured, and absolute and relative muscle thickness (percent total muscle thickness), order of thickness, and symmetry (percent difference between sides) were determined. Males had significantly larger muscles than females and size was poorly correlated with age. The pattern of relative muscle thickness was RA > IO > EO > TA. Symmetry for total absolute thickness of all three lateral muscles was 8%-9% (mean) but for individual muscles there was asymmetry of absolute size (13%-24%), whereas relative thickness was symmetrical for all muscles. These findings provide robust reference data for the abdominal muscles in normal males and females in order to enable comparison with clinical groups to assess abnormalities and establish sensitivity for evaluating the effectiveness of interventions.