Abstract: The present invention provides (1) a composition for improving pulmonary hypertension, comprising at least one substance capable of normalizing gut microbiota in a patient with pulmonary hypertension as an active ingredient; (2) a method for predicting the prognosis of a patient with pulmonary hypertension, or a method for assisting the determination of the severity of a patient with pulmonary hypertension, the method comprising detecting one or more types of bacteria selected from bacteria belonging to the family Micrococcaceae, Streptococcaceae, Pasteurellaceae, Veillonellaceae or Lactobacillaceae in gut microbiota in the patient with pulmonary hypertension; and (3) a method for assisting the diagnosis of pulmonary hypertension, the method comprising comparing the IgA level in feces of a subject to that of a healthy subject.