A patent ductus arteriosus in adults is a rare eventuality. After correct diagnosis, an accurate evaluation of pulmonary vascular resistances is necessary with, in case of normality, a straightforward indication to surgery, because the evolution of the disease cannot be predicted. In case of pulmonary hypertension irreversible pulmonary vascular damage must be excluded. Haemodynamic data such as pressures, flows and resistance ratios are not reliable. According to the majority of Authors the value of pulmonary resistances seems to be more accurate, and when more then 8 u/m2 an histological specimen must be obtained. In our experience a comparative study between the Health and Edwards method and the Yamaki et al., one shows, confirmed by postoperative follow-up, a greater reliability of the second one.