Isolated human monoclonal antibodies which bind to and inhibit human CD25, and related antibody-based compositions and molecules, are disclosed. The human antibodies can be produced by a hybridoma, a transfectoma or in a nonhuman transgenic animal, e.g., a transgenic mouse, capable of producing multiple isotypes of human monoclonal antibodies by undergoing V-D-J recombination and isotype switching. Also disclosed are pharmaceutical compositions comprising the human antibodies, nonhuman transgenic animals, hybridomas and transfectomas which produce the human antibodies, and therapeutic and diagnostic methods for using the human antibodies.