Human antibodies immunospecific for human CD27 are capable of blocking CD27 binding to its ligand CD70 and neutralizing bioactivity of CD27 including, but not limited to, CD27 intracellular signaling, T-cell proliferation and activation, B-cell proliferation and differentiation, plasmablast formation and alleviation of antibody responses, stimulation of tumor cells by CD70, and the production of soluble mediators from T and B-cells. The antibodies are useful in diagnosing or treating CD27 activity associated diseases and conditions.