The invention relates to a novel use of ultrafine nanoparticles, of use as a diagnostic, therapeutic or theranostic agent, characterized by their mode of administration via the airways. The invention is also directed toward the applications which follow from this novel mode of administration, in particular for imaging the lungs, and the diagnosis or prognosis of pathological pulmonary conditions. In the therapeutic field, the applications envisioned are those of radiosensitizing or radioactive agents for radiotherapy (and optionally curietherapy), or for neutron therapy, or of agents for PDT (photodynamic therapy), in particular for the treatment of lung tumors.
