Chromosome abnormalities were demonstrated in 50-100% of Giemsa-banded metaphases from nine cases of B-cell prolymphocytic leukemia (B-PLL). Mitoses were obtained with pokeweed mitogen following pretreatment of peripheral blood (PB) prolymphocytes with neuraminidase-galactose oxidase. Chromosome 14 was abnormal in eight of the nine cases: a marker 14q+, with breakpoint at band q32 in seven and trisomy 14 in one. In four cases the abnormal No. 14 was one of several primary abnormalities and in four others it was seen in secondary clones. The origin of the translocated material was unknown in three cases, in two it resulted from t(11;14), later becoming t(11;14;21) in one of them, t(1;14) in another, progressing later to t(1;14;17); in yet another patient, the 14q+ was the result of a complex rearrangement t(6;14;17). Abnormalities of chromosome 6 were seen in six cases: 6q- as the primary abnormality in three; trisomy 6 was part of secondary changes in one case. Structural abnormalities of chromosome 1 were seen in six cases: 1q- in four (in one as the only abnormality), 1q+ in one case, and 1p- in another, both in the main clone. Trisomy 12 was demonstrated in three cases but not as the primary change. Spleen cells in two patients showed a higher frequency of abnormalities than in the PB, supporting the concept of the spleen being the organ primarily involved in B-PLL. Evidence of karyotypic evolution was demonstrated in six patients, in some clearly associated with clinical progression of the disease. The type and frequency of the abnormalities observed in B-PLL resemble those seen in non-Hodgkin's lymphomas and suggest major differences from B-CLL, although a relationship with the latter can not be completely ruled out at present.