The current treatments for pulmonary arterial hypertension (PAH) target pulmonary vascular tone and attempt to address an imbalance in circulating and locally produced vasoconstrictors and vasodilators; namely restoring prostacyclin and cyclic GMP activity andinhibiting endothelin.New treatments for PAH are directed more at the structural vascular changes that characterize the condition.There is alsogreater interest in improving right ventricular function.This poses a challenge to the early assessment of efficacy-the important stage in drugdevelopment when decisions are made about investments in large scale clinical trials.First,drugs that act to reduce pulmonary vascular resistance byaltering vascular structure and address cardiac remodeling might be expected to take longer to demonstrate evidence of benefit than drugs that act onvascular tone.Second,there is the problem of measuring changes in vascular structure in vivo.Cardiac magnetic resonance is an excellent tool forassessing right ventricular mass and function and is increasingly used in clinical trials.Novel biomarkers are required to signal structural changes inblood vessels in response to a drug.Molecular imaging tools may offer a solution,particularly techniques used to answer questions on drug efficacyin oncology studies.