Introduction: The impact of infant physical activity (PA) on development is of particular interest to pediatric physical therapists who are tasked with supporting the acquisition of motor skills. In order to determine the role of infant PA on development, valid measurement of infant PA must be established. Purpose: The purpose of this study was to determine the validity of accelerometry to measure lower extremity PA in infants. Methods: Lower extremity PA was measured in twenty-three infants ages 2-4 months using a triaxial accelerometer placed on the infants’ ankle. Kinematic information was simultaneously collected via motion capture. Correlation coefficients were calculated to determine if a relationship existed between accelerometer measurement of infant PA and any of the kinematic variables. Results: No significant correlations were found between accelerometer of infant PA and kinematic descriptors of infant movement. Conclusions: Accelerometry may not be a valid measure of lower extremity infant PA. Future studies are necessary to determine if alternative data processing decisions would result in different results.