Abstract: A radially adjustable catheter apparatus and valve therefor is disclosed. The catheter apparatus used to provide percutaneous access to inner body cavities. The catheter apparatus is includes a tubular sheath, a tubular dilator extending through and beyond the sheath member and a flexible guide wire extending through the dilator. The sheath and dilator each extend from hollow housings, which interlock and fixedly retain the dilator within the sheath. The sheath further includes at least one pleat or fold along its longitudinal axis for expanding the diameter of the sheath. Disposed within the sheath housing are a hemostatic valve and a diaphragm, which effectuate hemostatic seals around the dilator and sheath, respectively. In a preferred embodiment, there is a pair of slidable plates, each having a semicircular aperture equal in diameter to the open end of the pleated sheath, affixed to the open end of the sheath member which is within the housing.