Humans have enjoyed forest environments for ages because of the quiet atmosphere, beautiful scenery, mild climate, and fresh, clean air. In the present study, we found that visiting forest parks, but not a city, enhanced human natural killer (NK) activity, increased anti-cancer proteins, such as perforin, granzymes A and B, and granulysin in NK cells, and reduced the level of stress hormones in both male and female subjects. Moreover, this effect lasted for more than 30 days after the trips, suggesting that visiting a forest park once a month would enable individuals to maintain a higher level of NK activity. Phytoncides released from trees and the decreased production of stress hormones may partially contribute to the increased NK activity. Because NK cells can kill tumor cells by releasing anti-cancer proteins, and visiting forest parks increases NK activity and the amount of anti-cancer proteins; therefore, the above findings suggest that visiting forest parks may have a preventive effect on cancer generation and progression.