Cancer antigen 125 (CA 125) is a glycoprotein expressed in normal tissues originally derived from celomic epithelia and its serum level is elevated in various benign and malignant conditions that involve stimulation of these tissues. Elevated levels have been reported in 40-43% of patients with non-Hodgkin's lymphoma (NHL) at diagnosis and were associated with parameters known to be associated with advanced and disseminated disease, and with event-free and overall survival. No difference in CA 125 level was found between indolent and aggressive lymphomas, and four of six patients with small lymphocytic lymphoma had elevated CA 125 levels. We therefore decided to measure serum CA 125 levels in chronic lymphocytic leukemia (CLL) patients, and evaluated them in 74 consecutive patients. The mean time from diagnosis to test was 74.5 months (range: 0-300). The mean serum CA 125 level was 16.3 U/ml (range: 3.7-133, normal value: <35 U/ml). CA 125 levels were elevated only in two patients (2.7%). To conclude, serum CA 125 levels are rarely elevated in CLL patients. It is possible that serum CA 125 levels can help differentiate between equivocal cases of small lymphocytic lymphoma and CLL.