Lymphoproliferative disorders of natural killer (NK)-cell lineage are well documented in humans but have yet to be documented in non-human primates (NHPs). Here we describe a case of NK-cell lymphoproliferative disorder/leukemia in a 20-y-old captive female rhesus macaque (Macaca mulatta). The animal clinically had mild splenomegaly and marked lymphocytosis with small-to-medium lymphocytes in blood smears. By flow cytometry and cluster differentiation, the lymphocytes were …