A novel, non-invasive technique for measuring water content in lungs is described. This technique could find important medical applications such as real time monitoring of lung edema development in patients affected by cardiac insufficiency undergoing physical effort. The technique is based on the moderation (speed decrease) of fast neutrons crossing the lungs, due mostly to the water contained in lungs. In the application of this diagnostic method, the necessary dose given to the patient is lower than that associated with lung imaging techniques that do not use an image intensifier. The results so far obtained are encouraging, and the method appears to be an improvement over those so far reported in literature as it is able to detect a lung water increase equivalent to 25% of a full edema with a 20% standard error. Research is being performed in order to refine the technique, investigate geometry effects and make the method suitable to the medical field.