Background : Heterophoria has been considered a disqualifying condition to flying performance because heterophoria, when exposed to high altitude hypoxia, becomes heterotropia causing double vision. However, there has been no actual evidence supporting this assumption. We investigated the binocular fusional ability and symptoms in the subjects having heterophorias upon exposure to acute hypobaric hypoxia. Methods : 15 subjects having heterophorias and 7 normal controls were included. All the subjects had exophoria and 1 subject had exophoria and hyperphoria. High altitude hypobaric hypoxic condition was simulated in a decompression chamber. Using Worth 4 dot test we measured the binocular fusional ability. To analyze the quality of binocular vision, we measured the stereoscopic visual acuity. By comparing the data between subjects and controls, we evaluated the binocular vision and fusional ability of subjects having heterophorias. Results: All the subjects and controls showed no disruption of binocular fusion on Worth 4 dot test upon exposure to hypoxia. On the whole, 7 of 15 subjects and 3 of 7 controls showed decreased level of stereopsis without significant differences between the two groups. There were no differences in binocular vision including fusional ability and stereopsis between the subjects and the controls in acute hypoxic condition. Conclusion: The subjects with heterophorias had stable binocular fusional ability resistant to acute hypobaric hypoxia, which showed no difference from the normal controls. Conclusively, we could not find any disadvantage in heterophoric persons with regard to flying performance. Visual standards for aviation personnel regarding heterophorias also need to be more lenient.