OBJECTIVES:To compare trunk muscle thickness of women with and without patellofemoral pain (PFP) and to assess the association of trunk muscle thickness with self-reported pain of women with PFP. METHODS:Forty-four women were recruited and divided into 2 groups: a PFP group (n = 22) and a pain-free group (n = 22). The thickness of the following trunk muscles was obtained by B-mode ultrasound imaging: transversus abdominis, obliquus internus (OI), obliquus externus (OE), rectus abdominis, and multifidus. Self-reported pain was measured on a visual analog scale. RESULTS:The 44 participants were 18 to 35 years old. Women with PFP had lower thickness of the OI and OE than pain-free women, with moderate or large effect sizes ranging from -0.78 to -0.98, which was negatively related to self-reported pain correlations (r = -0.53 to -0.40). The contraction ratios of the OI and OE were also lower in women with PFP than in pain-free women (P < .05). No differences between groups were found for the transversus abdominis, multifidus, and rectus abdominis, with also no correlation with self-reported pain. CONCLUSIONS:Lower thickness of the OI and OE is present in women with PFP, which is related to self-reported pain. These findings might help in understanding the alterations in trunk biomechanics of individuals with PFP and the mechanisms by which interventions targeting trunk muscle strength are beneficial to individuals with PFP.