In Japan, a trip to the forest, called "Shinrinyoku" in Japanese, is a short and pleasant visit to a forest; it is considered similar to natural aromatherapy. This review focuses on the effects of forest travel on human immune function. Since 2005, adult Japanese individuals, both male and female, have participated in a series of studies aimed at investigating the effect of forest baths on human immune function. Subjects experienced a 3-day / 2-night trip to forest areas and blood and urine were collected on days 2 (the first sampling during each trip) and 3 (the second sampling during each trip) and on days 7 and 30 thereafter. The travels. The activity of the natural killer (NK), the number of lymphocytes expressing NK, granulisin, perforin and granzimi A / B in the blood and the concentration of urinary adrenaline were measured. The same measurements were made before trips on a normal working day as a check. The mean values of NK activity and the number of cells expressing NK, granulisin, perforin and granzymes A / B in the forest bathing days were significantly higher than those of the control days, while the mean values of the urinary adrenaline concentration in the days of bathing in the forest were significantly shorter than those of control days in both male and female subjects. The increase in NK activity lasted for more than 30 days after the trip, suggesting that a trip to the forest once a month would allow people to maintain a higher level of NK activity. Conversely, a visit to the city as a tourist did not increase NK activity, the number of NK cells, or the level of intracellular granulisin, perforin, and granzymes A / B.