A novel crystalline zeolite composition is prepared by conducting the hydrothermal crystallization and, preferably, a preliminary aging step, at high pressure, i.e. pressures in excess of 20,000 psig (ca 138 MPa) and preferably above about 40,000 psig (ca 276 MPa megaPascals). The crystalline zeolite, designated zeolite HP, has a higher aluminum content than zeolite X and a lattice constant of above about 25.02 A.