We report a 58-year-old man who presented with fever, pancytopenia, hepatosplenomegaly, and "sinusitis" of his right nostril. Flow cytometric analysis of his bone marrow aspirate revealed a population of cells that were CD56+ (bright), CD2+ (dim), and CD7+ (slight brightly) but negative for CD3, CD4, CD5, CD8, CD11b, CD16, CD57, and T-cell receptors, consistent with aberrant natural killer cells. Bone marrow biopsy showed an atypical lymphoid infiltrate expressing CD56 as well as Epstein-Barr virus- encoded RNA and histiocytic hyperplasia with hemophagocytosis. Subsequent biopsy of his right nasal vestibule demonstrated an atypical lymphoid infiltrate, which was cytoplasmic CD3+, CD56+, and Epstein-Barr virus-encoded RNA positive, consistent with an extranodal nasal natural killer cell lymphoma. Conventional cytogenetic studies of the bone marrow revealed isochromosome 7q10 [i(7)(q10)] as the sole chromosomal aberration. To our knowledge, this is the first report demonstrating i(7)(q10) as a primary cytogenetic abnormality in an extranodal nasal natural killer cell lymphoma.