Knowledge of the precision with which the various ocular components can be measured with available techniques is vital to our ability to track changes in the anatomy of the eye in relation to the development of refractive error. Corneal touch A-scan ultrasound measurements were obtained by either an instrument-mounted or by a hand-held method on 469 children aged 6 to 11 years at their enrollment into the Correction of Myopia Evaluation Trial. Variability of measurements was calculated for overall axial length, anterior chamber depth, lens thickness, and vitreous chamber depth. The mean variability of overall axial length was 0.062 +/- 0.043 mm and 0.061 +/- 0.056 mm for the right and left eyes, respectively. Mean variability of the ocular component measures was similar. The mean variability was not influenced by the age, gender, level of myopia, or residual accommodation of the subjects. Statistically significant differences between the instrument-mounted and the hand-held method and among certain ethnic groups were found, but the differences were not of a magnitude to be clinically significant. A-scan ultrasonography is sensitive to changes in the axial length and vitreous chamber depth equivalent to 0.25 D and is therefore a useful technique to assess changes in these ocular components in children. The precision of lens thickness is poorer than the equivalent of 1.0 D, and, therefore, A-scan may not be sufficiently precise to be useful in studies of active accommodation or lens growth.