Objective: To measure the effect of orthokeratology lenses on ocular biometric parameters in children with low to moderate myopia. Methods: In this randomized controlled trial, 100 children with low to moderate myopia with an average age of 11.0士1.9 years were recruited at the clinical center of Beijing
Tongren Hospital.  Subj ects were randomly assigned to an orthokeratology lens group or a spectacles group.Cycloplegic refraction, axial length (AL), central corneal thickness (CCT), anterior chamber depth (ACD),and crystalline lens thickness (CLT) were measured at baseline and at six months. One year later, the
children in the orthokeratology group were asked to pause wearing the lenses for one month, after which the measurements were repeated. Data were analyzed by repeated measures analysis of variance and by independent t-tests. Results:  Nine subj ects dropped out from the trial. There were no statistically significant
differences between the spectacle (46 cases) and the orthokeratology (45 cases) groups at baseline or for characteristics such as age, gender, cycloplegic refraction, AL, CCT, ACD, and CLT. The increase in AL for the orthokeratology group was slower than that in the spectacles group both at 6 months (0.10 mm vs.
0.21 mm, t~3.213. P=0.002). Following the one month pause in orthokeratology lens wear after one year of wearing them, the increase in AL of the orthokeratology group, 0.23 mm, was smaller than that of the spectacle group, 0.34 mm (t-2.388, P=0.019). For children in the orthokeratology group, after six months
of lens wear. the CCT and ACD decreased by 4.80 ym and 0.03 mm (t~6.284. P<0.001; z~4.473, P<0.001),respectively, and the CLT increased by 0.02 mm (t=3.971, P<0.001) compared to baseline. After the one month pause following the year of lens wear, CCT rebounded by 3.45 ym, while the ACD decreased by
0.01 mm (z~2.829, P=0.006) and CLT decreased by 0.03 mm(F～3.589, P<O.O01), respectively. Except for CCT, the changes were all significantly different from those in the spectacles group. Conclusions:Orthokeratology can have large and long-lasting effects on controlling axial elongation, decreasing ACD,
and increasing CLT. The effects remained after a one month pause of orthokeratology lens wear.