Pulmonary arterial hypertension (PAH) associated with systemic lupus erythematosus (SLE) or mixed connective tissue disease (MTCD), in contrast to other types of PAH, may respond to immunosuppressive therapy. Most PAH cases with an immunosuppressant response were in the early stages of the disease (WHO functional class III or less). The present case was a 34-year-old woman with MCTD-associated PAH (WHO functional class IV) who was resistant to a combination of three vasodilators. Afterwards, she was treated with glucocorticoid and cyclophosphamide. This case suggested the potential benefit of immunosuppressants in patients with severe MCTD-associated PAH.