Cytogenetic studies on lymph node and skin biopsy specimens and peripheral blood in 104 patients with peripheral T-cell lymphomas (PTL) were compared with histopathologic diagnoses made according to the updated Kiel classification. Low-grade lymphomas presented normal metaphases more frequently than high-grade ones (P < .0001). This difference remained significant if cases with greater than 10% and greater than 50% normal metaphases in unstimulated cultures and in cultures stimulated by different mitogens were compared. On the other hand, high-grade lymphomas more often showed aberrant clones (P < .05), triploid to tetraploid clones (P < .0001), and complex clones with more than four chromosome changes (P < .01). Low-grade PTL showed consistent cytogenetic features. Clones with both inv(14)(q11q32.1) and trisomy 8q, mostly caused by i(8q)(q10), were found in all cases of T-cell chronic lymphocytic leukemia (T-CLL) and T-cell prolymphocytic leukemia (T-PLL). Trisomy 3 was observed only in angioimmunoblastic lymphadenopathy with dysproteinemia (AILD)-type PTL, T-zone lymphoma, and lymphoepithelioid lymphoma. Moreover, the proportion of normal metaphases in these PTL was higher than in the other low-grade PTL (P < .01). On the contrary, T-CLL, T-PLL, and cutaneous T-cell lymphomas (CTCL) showed complex clones (P < .0001), duplications in 6p (P <.01), deletions in 6q (P < .01), trisomy 8q (P < .00001), inv(14) (P < .00001), and monosomy 13 or changes of 13q14 (P < .001) more frequently than the other low-grade PTL. Trisomy 5 and + X predominated in AILD- type PTL. A cytogenetic feature characteristic of AILD-type PTL and CTCL was unrelated clones, which were found in 15% of AILD-type PTL and 17% of CTCL. The only chromosome aberration restricted to a certain high-grade PTL was t(2;5)(p23;q35) in large-cell anaplastic lymphoma. Deletions in 6q, total or partial trisomies of 7q, and monosomy 13 or changes of 13q14 turned out to be significantly more frequent in high- grade than in low-grade lymphomas (P < .01, P < .01, and P < .05, respectively). In summary, the cytogenetic findings in our series of 104 PTL enabled us to distinguish not only between low-grade and high- grade lymphomas but also between various entities of PTL. Thus, the cytogenetic findings paralleled the histopathologic diagnoses made according to the updated Kiel classification.