The detachable balloon catheter assembly comprises a balloon and sealing valve assembly including a sealing valve being formed of a resilient material having an elongate passageway extending therethrough and being mounted in a sleeve, an inflatable balloon having a mouth portion which is bonded to the sealing valve, and a small diameter cannula having a distal end which extends through the passageway in the sealing valve. The small diameter cannula includes a connector terminal on the proximal end which is adapted to be coupled to a source of fluid pressure. The passageway in the sealing valve takes the form of an elongate slit prior to insertion of the small diameter cannula through the passageway, and upon insertion of the cannula through the passageway, the passageway takes the form of a cylindrical aperture which is in fluid-tight engagement with the outer surface of the cannula while allowing the cannula to easily slide through the passageway. When the balloon is inflated to a desired size, the cannula may be withdrawn from the passageway in the sealing valve thereby causing the passageway to revert to the slit configuration in order to provide a fluid-tight seal for the inflated balloon. In one embodiment, a piston is mounted on the small diameter cannula and an aperture extends through the side wall of the cannula so that a burst of fluid pressure may be applied to the piston causing it to be driven away from the sealing valve to drive the cannula out of engagement with the sealing valve for detachment of the cannula from the inflated sealed balloon.