In this study, we describe the cytological and cytogenetic features of six Epstein‐Barr virus (EBV)‐infected natural killer (NK) cell clones. Three cell clones, SNK‐1, ‐3 and ‐6, were derived from patients with nasal T/NK‐cell lymphomas; two cell clones, SNK‐5 and ‐10, were isolated from patients with chronic active EBV infection (CAEBV); and the other cell clone, SNK‐11, was from a patient with hydroa vacciniforme (HV)‐like eruptions. An analysis of the number of EBV‐terminal repeats showed that the SNK cell clones had monoclonal EBV genomes identical to the original EBV‐infected cells of the respective patients, and SNK cells had the type II latency of EBV infection, suggesting that not only the cell clones isolated from nasal T/NK‐cell lymphomas but also those isolated from CAEBV and HV‐like eruptions had been transformed by EBV to a certain degree. Cytogenetic analysis detected deletions in chromosome 6q in five out of the six SNK cell clones, while 6q was not deleted in four control cell lines of T‐cell lineage. This suggested that a 6q deletion is a characteristic feature of EBV‐positive NK cells, which proliferated in the diseased individuals. The results showed that EBV‐positive NK cells in malignant and non‐malignant lymphoproliferative diseases shared common cytological and cytogenetic features.