A 79-year-old woman was admitted to our hospital for an investigation of a large 13-cm tumor in the chest and treatment for dyspnea in January 2010. The tumor had been observed on chest X-rays since 1992. It had measured 7 cm in 2008, then started to grow rapidly. Further investigations revealed that it was a malignant solitary fibrous tumor that was strongly suspected to have transformed from a benign to malignant state. Resection was not possible, and the patient died one month later. Benign solitary fibrous tumors of the pleura may become malignant during long-term follow-up. All suspected or proven solitary fibrous tumors of the pleura should be resected.