Objective: To describe gender and side-to-side differences in lateral abdominal wallmuscle thickness at rest and during contraction in asymptomatic adults.Design: Cross-sectional observational study.Setting: Tertiary university outpatient musculoskeletal clinic.Patients: Asymptomatic, healthy adults.Methods: A single examiner with training in ultrasound (US) assessment evaluated allsubjects and used a different hand to guide the US transducer on each side. Bilateralanteroposterior measurements of transversus abdominis (TrA) and internal oblique (IO) thickness were assessed at rest and during maximal contraction with the use of an abdominal draw-in maneuver (ADM). Three sets of data points were obtained on 12 subjects to determine intrarater reliability within a single session. Main Outcome Measurements: Anteroposterior thickness of TrA and IO muscles (cm). Results: A total of 20 subjects (mean age, 29.3 years; range, 23-46 years; 8 men and 12 women; mean body mass index, 24.4 kg/m2; range, 18.9-34.9 kg/m2) were examined. Atrest, men had greater TrA and IO thickness compared with women (men, 0.55, and women,0.35, P = .01; and men, 0.95, and women, 0.77, P= .04, respectively). At rest, there were no side-to-side differences of TrA or IO muscles for both genders. With an ADM, womendemonstrated a significant percent change in thickness of their TrA muscle compared with men (women, 0.77, and men, 0.29, P= .001). No significant difference was found between men and women in the percent change of IO thickness with ADM (women, 0.20, and men,0.12, P = .24). There was excellent intrarater reliability for measurements of the right abdominal muscles (intraclass correlation coefficient = 0.92-0.95) and moderate-to-goodintrarater reliability for measurements of the left abdominal muscles (intraclass correlationcoefficient = 0.62-0.92). Conclusions: Asymptomatic men have greater TrA and IO thickness at rest compared with asymptomatic women; however, women demonstrated a greater percent change in TrA thickness with an ADM compared with men.