Posttraumatic malalignments are a frequent sequlae of IM nailing of lower extremity fractures. Conventional US has proven to be inferior to CT determinations of tibial or femural length and torsion. A new 3-D US method is presented that allows for accurate single step determination of lower extremity length and torsion without ionizing radiation. A regular US machine with a 5 Mhz linear probe is combined with an US localizer. Reference markers affixed to the lower extremity eliminate errors associated with patient position or motion. The 3-D US method was compared against CT (Ulm's method) in the measurement of torsion and length of the tibia and femur in 50 adults and 50 children. In both methods, the maximum difference of the intraindividual torsional angles and length measurements was 7 degrees and 7 mm. The maximum standard deviation for reproducibility in length measurement was 1.6 mm and 1.5 degrees for angular torsion. The new 3-D US technique was superior to CT in terms of reliability and reproducibility. Clinical advantages of the 3-D US technique include rapidity, independence from patient motion or positioning and the avoidance of ionizing radiation. Indications for 3-D torsional and length determinations include follow-up evaluation of adult and pediatric tibial and femoral fractures, pediatric limb and gait evaluations, and osteotomy planning.