Since 2004, we have conducted a series of studies of the effect of forest therapy on human health and established forest therapy as a new preventive strategy. We have found that forest therapy has many beneficial effects on human health. However, there is almost no study dealing with the possibility of clinical applications of forest therapy. In this review, we discuss the possibility of clinical applications of forest therapy from the following viewpoints: 1. Forest therapy can decrease blood pressure, heart rate, sympathetic nerve activity, and levels of stress hormones, such as urinary adrenaline and noradrenaline, and can increase parasympathetic nerve activity, suggesting its preventive effect on hypertension. 2. Forest therapy can also decreace the scores for anxiety, depression, anger, fatigue, and confusion and increase the score for vigor in the Profile of Mood States (POMS) test, suggesting its preventive effect on mental depression. 3. Forest therapy can increase the activity and number of human natural killer (NK) cells and the intracellular levels of anticancer proteins, suggesting its preventive effect on cancers. 4. These findings suggest that forest therapy may have preventive effects on lifestyle-related diseases. However, the above preventive effects of forest therapy should be confirmed in clinical research.