The aim was to evaluate the effectiveness of strengthening exercises using the Swiss ball in patients with fibromyalgia through a randomized controlled trial with intention to treat analyses. A total of 60 patients with fibromyalgia met the inclusion criteria and were randomly allocated to either the Swiss ball exercise group (n=30) or a stretching group (n=30). All patients participated in 40-minute training sessions 3 times per week for 12 weeks. Pain (Visual Analogue Scale 0-100); muscle strength (One Repetition Maximum test); health status (Fibromyalgia Impact Questionnaire Revised); quality of life (Short Form-36 questionnaire) were evaluated at baseline, and after 6 and 12 weeks of training. The Swiss ball group showed a statistically significant improvement in VAS (0-100) (p<0.001), SF-36 (p<0.05) and Fibromyalgia Impact Questionnaire (p<0.001) compared with the stretching group. The results of this study proved that the treatment for fibromyalgia with strengthening exercises and the use of the Swiss ball led to improvement of pain, quality of life, muscle strength and decreased the need for medications for this disease compared to stretching exercises, without negative effects.