Abstract: A kidney-ureter catheter unit comprises a flexible catheter (2) provided with an introduction end (2a) of truncated conical form close to which radial drainage holes (3) are pierced. Within catheter (2) is slidably engageable a semi-rigid dilator mandrel (4) provided with an introduction end (4a) of truncated conical form and in turn slidably engaged by a semi-rigid guide element (12) having a rounded introduction end (12a). A spring (5) detachably fastens mandrel (4) to catheter (2) in an insertion condition with respect to each other in which the introduction ends (2a, 4a) of catheter (2) and mandrel (4) have the respective generators in alignment, so as to form a continuous frusto-conical surface.