Estrogen receptor (ER) determinations were performed on cytosol preparations of Ficoll-Hypaque density separated mononuclear cells from 11 patients with chronic lymphocytic leukemia (CLL). The presence of ER was noted in 8 of 11 specimens (73%). ER ranged from 431 fmole/mg to 4.3 fmole/mg cytosol protein. Two types of receptor subunits were observed at the 8S and 4S region of the sucrose gradient. In addition, 1 of 3 Epstein-Barr virus (EBV) transformed B-lymphoblastoid cell lines from healthy donors had a measurable amount of ER. Patient R.L., who was refractory to standard chemotherapy and radiation and was ER positive, experienced a minor response to Tamoxifen therapy, with subsequent loss of ER. The demonstration of ER in CLL suggests that this malignancy may have a hormone-dependent subpopulation of cells.