Fresh tumor samples from 27 patients with large cell lymphoma, either previously untreated (26 patients) or minimally treated (one patient), were processed for cytogenetic studies. Cytogenetic abnormalities were observed in all patients, most commonly in chromosomes 1, 3, 7, 12, 14, 17, and 18. Nine chromosomal breakpoints appeared frequently: 14q32 in 14 instances; 18q21 in seven; 9p13-21, 17p11-13, and 3q21-23 in six each; 1p11-21 in five instances; 1p36 in four; and 2p21-23 in three. The most common structural abnormalities were t(14;18)(q32;q21) in seven patients (26%) and 17p- in six (22%). The presence of 17p- was associated with a significantly higher proliferative capacity as manifested by the percentage of S phase = 22% versus 11% for cases without 17p-(P less than 0.05). Trisomy 12, typical of small lymphocytic lymphoma, was seen in five patients in this series, all of whom had diffuse large cell lymphoma; frequently, it appeared simultaneously with t(14;18). The two patients with immunoblastic lymphoma of B-cell type had an abnormality involving chromosome 2p21-23. Deletions in the long arm of chromosome 6, previously described as typical of diffuse large cell lymphoma and B-cell immunoblastic lymphoma were observed infrequently in this series. However, this abnormality has been present in 50% of patients with large cell lymphoma previously exposed to therapy, suggesting that it may be related to effects of chemotherapy or to clonal evolution.