Cellular heat shock proteins (hsp) are synthesized mainly in response to stress, e.g. heat, or oxidative stress and are differentially expressed in malignancies. High constitutive expression of hsp 90 in patients with acute leukemia has been associated with celular proliferation and was suppressed after the induction of differentiation. The constitutive expression of the genes for four heat shock proteins (HSPs) was studied in cells obtained from 13 patients with chronic lymphocytic leukemia and normal blood cells by means of Northern-blot analysis and RT-PCR. Patients were classified according to the RAI classification. Three patients were classified as stage I, five patients as stage II and five patients as stage IV. HSP 60, HSP 70, HSP 90 but not HSP 27 mRNA were expressed in MNC cells from healthy volunteers as determined by RT-PCR. HSP 70 and HSP 90 were expressed in all but one patient, wheras HSP 27 and 60 were more frequently expressed in patients with stage IV disease. Additional cases of patients with CLL should be examined to determine whether the expression of HSP 27 and 60 is confined to different stages of the disease.