PURPOSE: We determined the reliability of real-time ultrasound for detecting involuntary or reflex pelvic floor muscle contraction. MATERIALS AND METHODS: A total of 118 women with the symptomatic and urodynamic evidence of stress urinary incontinence were recruited in a 3-year period from tertiary referral urogynecology clinics. Under the oral instruction of strong coughing involuntary pelvic floor muscle contraction was detected using introital ultrasound in real-time and cine loop modes. Two dynamic imaging features were considered involuntary pelvic floor muscle contraction, including an inward clitoral motion and a superior-anterior anorectal lift before or during coughing. RESULTS: Real-time ultrasound had good reliability to detect involuntary pelvic floor muscle contraction. For interobserver and intra-observer reliability Cohen's kappa was 0.645 to 0.679. Of the 118 women real-time ultrasound before or during coughing showed an inward clitoral motion in 100 (93%) and an anorectal lift in 108 (92%). Neither type of contraction during coughing was identified in 2 women. Although the 2 contraction patterns were synchronized in 88% of women, for this synchronization kappa was only 0.159, indicating poor coordination between these 2 involuntary activities. CONCLUSIONS: Real-time ultrasound is a reliable tool for detecting involuntary pelvic floor muscle contraction.