Atrophy of the stabilizing muscles of the hip, such as quadratus femoris (QF), may predispose the hip joint to dysfunction. Ultrasound imaging (USI) has been shown to be a valid and reliable method of measuring the size of several hip muscles, but QF size has not been investigated. The objectives of this study were to establish the criterion validity of USI against the "gold standard" magnetic resonance imaging (MRI) for measuring the QF cross-sectional area (CSA) and to investigate intra-rater reliability of USI. Eleven current or retired professional ballet dancers (six women, five men) volunteered for USI of their QF within 1 week of MRI. The mean CSAs of QF were compared between the two imaging modalities, and the mean USI CSAs from two different trials were compared. Mean CSA with MRI (4.8 cm(2), ± 1.54) was significantly larger than mean CSA with USI (4.29 cm(2) ± 1.56; t = 5.82; p < 0.001), and the mean difference was 0.41 cm² (9%). However, the measures were highly correlated for intra-class reliability (r = 0.96, p < 0.001), and intra-class correlation coefficients (ICC) demonstrated excellent agreement (ICC = 0.90; 95% confidence interval [CI]: 0.20 -0.97) and consistency of measures (ICC = 0.96; 95% CI: 0.90- 0.98). Intra-rater reliability of measuring QF with USI was excellent between two trials (ICC = 0.98; 95% CI: 0.96- 0.99). The minimal detectable change at a 95% CI (MDC95) was 0.38 cm(2) (9.5%). It is concluded that USI is a valid and reliable measure of QF muscle size and can be used to measure QF CSA in a research or clinical setting.