Design: In this double-blind , prospective clinical study, subjects suspected to have an acute stress fracture of the lower extremity will be recruited from Sports Medicine Clinic (conducted by two physicians boarded in Physical Medicine and Rehabilitation and one physician board in Family Medicine and all boarded in Sports Medicine). When patients present to one of three physicians in Sports Medicine Clinic with a suspected acute stress fracture of the lower extremity, they will be offered the opportunity to enroll. Subjects will be required to have already undergone the initial step of the traditional diagnostic algorithm with x-ray of the suspicious region on the day of enrollment. All subjects will then be scheduled to undergo diagnostic ultrasound at a separate appointment with ultrasound procedure performed by a blinded clinical physician boarded in Physical Medicine and Rehabilitation, competent in performing and reading diagnostic ultrasound. All patients will then undergo a confirmatory MRI of the region of concern. Subject recruitment and data collection are expected to be completed over a six-month period. A statistician in the Department of Public Health at the University of Virginia will be performing statistical analysis. We plan to analyze findings using a McNemar chi-square test to evaluate for significant differences between the sensitivities of ultrasound and x-ray in diagnosing acute stress fracture of the lower extremity. All x-ray and MRI obtained for this study will be read by a Radiologist board certified in Musculoskeletal Radiology.