The purpose of this study was to investigate the changes in patellar tendon length during passive knee flexion in the menstrual cycle. The right knee joints of 10 adult women with stable menstrual cycles were evaluated. Measurement of patellar tendon length was performed when the knee joint was bent flexibly at six angles (0°, 30°, 60°, 90°, 110°, and 130°), and the long-axis image of the patellar tendon was transversely superimposed at each angle. A photograph was taken with an ultrasonic device. Measurements were performed once during each of the four phases of the menstrual cycle (menstrual, follicular, ovulatory, and luteal phases) classified by the basal body temperature method. There were no significant differences in patellar tendon length at each knee flexion angle in each phase of the menstrual cycle. In the luteal phase, the patellar tendon was stretched significantly at 90° and 130° compared to knee flexion of 0°.