Aim: To determine the effect of human in vivo accommodation on the stability of the crystalline lens. Methods: Using a dual Purkinje image (DPI) eyetracker, the phase-time difference and amplitudes of Purkinje images I (P-I) and IV ( P-1V) were measured in 37 normal emmetropic subjects ( 34 women and 3 men; mean age 19.8, range 18-22 years) when they changed focus from 70 to 15 cm and simultaneously rotated their heads horizontally from side to side or made horizontal saccades between two targets 6.8 apart. Results: When the subjects changed focus from 70 to 15 cm and rotated their heads or made eye saccades, the phase-time difference between P-I and P-IV decreased. During saccades, the amplitude of both PI and PIV overshoots significantly increased with focus at 15 cm; however, their ratio (P-IV overshoot amplitude/P-I overshoot amplitude) significantly declined. Conclusions: The lens is stable during accommodation. The implications of these findings on the mechanism of accommodation are discussed.