Purpose To determine the accuracy of US for the quantitative parameters of cam-type femoroacetabular impingement (FAI) in an asymptomatic adult population, using MRI as the reference standard. Materials and methods We prospectively evaluated 158 hips of 79 adult subjects (44 males, 35 females; age range 18-49 years) with a negative anterior impingement test. Two observers independently measured anterosuperior quadrant alpha angles and anterior femoral distances (AFDs) on both US and MRI. Results US measurements of alpha angle and AFD had a moderate-to-high positive correlation with MRI measurements and there was substantial and/or almost perfect agreement between observers on both imaging modalities. Anterior femoral distance measurements for both observers and alpha angles at MRI for Observer 1 showed no statistically significant difference between genders. Conclusion Ultrasonography can be used for anterosuperior alpha angle and AFD measurements in asymptomatic adults. However, when used as the sole imaging method, its inherent limitations and possible discrepancies with MRI should be kept in mind.