AbstractPurpose: To evaluate the influence of the menstrual cycle on parenchymal strain in ultrasound elastography of the breast. Subjects and methods: Ultrasound elastography was used to examine the 40 breasts of 20 healthy premenopausal women volunteers (mean age, 31.4 years; age range, 24 to 46 years). Each of the women had a regular menstrual period. We determined the values of the subcutaneous fat-parenchymal strain ratio by ultrasound elastography three different times: at the beginning and end of the menstrual period and again during the ovulation period. On each examination, we measured the strain ratio in each breast, obtaining five repeated measurements in the fixed direction. Results: We used the Freidman assay for the statistical analysis of the median and average values of every subcutaneous fat-parenchymal strain ratio. No significant differences were found among the vales of subcutaneous fat-parenchymal strain ratio at the beginning and end of the menstrual period, and the ovulation (average: p=0.36, median: p=0.33. Conclusion: Ultrasound elastography is not influenced by the menstrual cycle.