Most patients with fibromyalgia benefit from different forms of physical exercise. Studies show that exercise can help restore the body's neurochemical balance and that it triggers a positive emotional state. So, regular exercise can help reduce anxiety, stress, and depression. The aim of this study was to analyze the benefits of moderate aerobic exercise when walking in two types of forests, young and mature, and to assess anxiety, sleep, pain, and well-being in patients with fibromyalgia. Secondary objectives included assessing (i) whether there were differences in temperature, sound, and moisture, (ii) whether there was an improvement in emotional control, and (iii) whether there was an improvement in health (reduction in pain) and in physical and mental relaxation. Patients and Methods. A study involving walking through two types of forests (mature and young) was performed. A total of 30 patients were randomly assigned to two groups, mature and young forests. The participants were administered the following tests: the Spanish version of the Revised Fibromyalgia Impact Questionnaire (FIQR) at baseline and the end-point of the study, the State-Trait Anxiety Inventory (STAI) after each walk, and a series of questions regarding symptomatic evolution. Several physiological parameters were registered. Results. FIQR baseline and end-point scores indicated a significant decrease in the symptomatic subscale of the FIQ (SD = 21.7; ; ). The within-group analysis revealed that differences were significant with respect to days of intense pain, insomnia, and days of well-being only in the group assigned to the mature forest, not in the group assigned to the young forest. No differences were found with respect to anxiety. Conclusions. Although the main aim of this research was not achieved, as the results revealed no differences between the groups in the two forest types, authors could confirm that an aerobic exercise program consisting of walking through a mature forest can provide the subjective perception of having less days of pain and insomnia and more days of wellness, in patients with fibromyalgia