OBJECTIVE:: To evaluate the effect of room illumination on the measurement of anterior segment parameters. METHODS:: In this comparative study, measurements of anterior segment parameters of 25 eyes of 25 healthy patients were performed by optical low-coherence reflectometry (OLCR). Measurements were taken under photopic conditions (150 lux) and under mesopic conditions (3 lux). Paired t test by SPSS 16.0 was used to compare the anterior segment measurements performed in both conditions. A P value lower than 0.05 was accepted as statistically significant. RESULTS:: The mean age of the patients was 27.09±1.72 years (range, 20-40 years). Of the anterior segment parameters, axial length and keratometry did not show significant difference between photopic and mesopic conditions (P=0.541 and P=0.812, respectively). The mean anterior chamber depth measurements was 3.04±0.35 mm and 3.06±0.35 mm, whereas the mean lens thickness was 3.75±0.36 mm and 3.77±0.36 mm in the mesopic and photopic conditions, respectively. The mean pupil diameter measurement was 4.86±0.70 mm under photopic conditions and 6.36±0.94 mm under mesopic conditions. The anterior chamber depth, lens thickness, and pupil diameter showed significant difference between photopic and mesopic conditions (P=0.01, P=0.006, and P=0.0001, respectively). CONCLUSION:: Anterior chamber depth, lens thickness, and pupil diameter were affected by the changes in luminance while performing measurements with OLCR. Considering the age of the study group, further studies are needed to test the measurement of accommodation amplitude. © 2014 Contact Lens Association of Ophthalmologists.