PURPOSE: To compare the success rates of eye drop instillation in the sitting position and supine position among Japanese patients with ocular diseases (cataract, glaucoma, or retinal and vitreous diseases). METHODS: Patients who were hospitalized in Okayama University Hospital for eye surgery were studied. Instillation procedures of each patient in both the sitting and supine positions were recorded using a video camera at the time of instillation. We defined "success" when one drop fell accurately onto the ocular surface at the first attempt. Instillation of two or more drops, drops delivered to a site other than the eye surface, and touching the eyelashes, eyelids, or conjunctiva with the tip of the eye drop bottle were regarded as "failure". We excluded patients with vision below counting finger. RESULTS: One-hundred and two patients (54 males and 58 females, aged 70.2 ± 12.3 years) with ocular disease who were hospitalized for surgery (cataract: 61.8%, glaucoma: 15.7%, retinal and vitreous diseases: 22.5%) were included in this prospective observational study. The mean duration of eye drop use was 3.1 ± 5.2 years. The success rate of eye drop instillation was significantly higher in the supine position than in the sitting position (64.7% vs. 50%, P = 0.0039). The mean age was significantly higher in the failure group than in the success group (74.0 ± 11.5 vs. 67.7 ± 12.4 years, P = 0.0085) for the sitting position, but not significantly different for the supine position (72.3 ± 12.9 vs. 70.1 ± 12.0 years, P = 0.3849). No significant differences in mean duration of drop use, mean corrected VA, and mean spherical equivalent refraction were observed between success and failure groups, for both sitting and supine positions. CONCLUSIONS: In the present study, the success rate of eye drop instillation was significantly higher when applied in the supine position than in the sitting position.