Is connective tissue disease (CTD) associated pulmonary arterial hypertension (CTPAH) a distinct entity or group of conditions, or merely a form of idiopathic pulmonary arterial hypertension (iPAH) perhaps triggered by the immunological perturbations seen in these conditions? In arguing for causality as is recognized for HIV-associated pulmonary arterial hypertension (PAH), fulfilling Hill’s criteria for causation is quite complex,1 especially because failure to meet any of the criteria does not preclude a cause-effect relationship.