Our ophthalmologists should have a sense of wonder for IOP. IOP is well-organized physiological factor to maintain ocular function. If it goes up, the glaucoma may happen, whereas if it goes down, the hypotensive retinopathy or the choked disc may happen. IOP is always fluctuating through the day and the season. The seasonal variation of IOP indicating the higher pressure in the winter season has been well-known physiological variation of IOP. In addition, age, body position, and combined diseases may affect IOP. Therefore, call of unique value of IOP for the eye is a big challenge. Ikeda and Mori reported the seasonal variation and the trend of IOP based on the longitudinal data obtained at the outpatient clinic over a 20-year period in NTG patients. It was found that IOP gradually decreases by age with the seasonal variation even the patients were medically treated. This conclusion based on 50000 of data points in 1774 NTG patients is robust. This information is useful in the outpatient clinic for explaining the variation of IOP or in the study protocol for considering the seasonal changes. Although the age-related decrease of IOP may be not so clinically important, it is interesting that the seasonal changes are sustainable even in the patients who controlled fully in the low pressure by glaucoma medications. Because we cannot have the data of the normal and the untreated NTG patients such as this longitudinal data, we do not identify whether this year-trend or the seasonal changes are specific for Japanese NTG patients or not. In addition, the effect of other variation factors such as body position and diurnal variation are also interesting topics in these subjects. In the future, the mechanism of IOP regulation and variation should be clarified and the appropriate treatment to suppress the IOP variation will be developed.