Aim: To assess differences in axial length of the eye between young adults aged 20 to 39 years living in urban and rural environments in central Mongolia. Participants and methods: Multi-stage, clustered, simple random sampling was used to identify 375 people living in the capital city (Ulaanbaatar) and 375 in an adjacent rural district (Selenge aimag). Axial length was measured using A-mode ultrasonography. Height and weight were recorded. Socio-economic data were obtained using a questionnaire. Results: 568 participants (75.7%) were examined. Mean axial length was 23.35 mm (95% confidence interval, 23.27-23.43 mm). Participants in the urban area were 2 cm taller and 3 kg heavier than the rural population, and 5 times as likely to have been educated to college level (p < 0.001). The ratio of participants in the rural group reporting a very low income to those in the urban group was more than 3:1 (p < 0.001). Axial length was significantly longer in the urban participants (23.53 mm; SD, 1.03 mm) than in the rural participants (23.19 mm; SD, 0.90 mm) [p < 0.001]. This difference was apparent in the age groups of 20 to 29 years and 30 to 39 years. Mean axial length was significantly longer in people aged 20 to 29 years (23.66 mm; SD, 1.13 mm) compared with those aged 30 to 39 years (23.37 mm; SD, 0.85 mm) [p = 0.03] in the urban group, but the same difference was not present in the rural group. Multivariate analysis identified height (p < 0.001) and educational achievement (p < 0.001) as being significantly associated with axial length. Urban or rural residence was of borderline significance (p = 0.054). Conclusions: These data indicate that axial length is significantly longer in city-dwelling young adults in Mongolia. In part, this is attributable to their greater height. The link with educational achievement may reflect the recognised association with near-work and myopia. It seems likely that other important associations explaining axial length differences between urban and rural populations remain to be identified.