This 5-year prospective follow-up study investigated whether short axial length influences the incidence of new idiopathic macular holes in fellow eyes of patients with unilateral macular hole. A total of 100 unilateral idiopathic macular hole patients and age- and sex-matched controls were followed for 5 years. The incidence of macular hole in the fellow eye was prospectively observed, and the mean axial length was compared between fellow eyes that developed new macular holes and those that did not. During the follow-up, 11 of 100 fellow eyes in unilateral macular hole patients developed a new macular hole, whereas no cases occurred in the control group. The mean axial length of fellow eyes with new macular hole was 21.42 ± 0.39 mm, significantly shorter than that of fellow eyes without new macular hole, which measured 22.98 ± 0.78 mm (p < 0.001). These findings demonstrate that short axial length is significantly associated with the development of new idiopathic macular holes in fellow eyes and provide prospective evidence supporting short axial length as a risk factor. Given its accessibility and practicality in clinical practice, axial length may serve as a useful parameter for identifying high-risk patients and guiding follow-up strategies.