OBJECTIVE To evaluate diurnal variation of higher order aberrations in human eyes. METHODS A prospective study of 20 myopic eyes was performed. Cycloplegic aberrometry (Zywave) and refraction were measured at 2-hour intervals for five time points beginning at 10:00 AM. Changes in measured magnitude for third and fourth order Zernike coefficients were compared over time. Analysis of variance (ANOVA) was performed to test for statistically significant differences across time points for each Zernike coefficient. A separate prospective study was performed of 11 myopic eyes to determine and compare diurnal variation with the repeatability coefficient of individual Zernike coefficients measured using the Zywave aberrometer. RESULTS Mean refraction was -0.75 +/- 1.25 diopters (D) sphere and -0.57 +/- 0.43 D cylinder. No statistically significant change (ANOVA) was noted over the period of observation for any individual Zernike coefficients. Only Z3(-3) (vertical trefoil) showed an average increase of 0.029 microm (P = .98) towards the end of the day; however, this trend was not statistically significant. The standard deviation of individual Zernike coefficients ranged from 0.035 microm (Z4(4)) to 0.165 microm (Z3(-1)). CONCLUSIONS No statistically significant diurnal variation of higher order aberrations was measured by Zywave aberrometer. The trend of Z3(-3) increasing later in the day was most likely a Zywave measurement error artifact.