We have investigated the role of immunophenotyping in distinguishing between leukemic B-cell lymphoproliferative disorders. Circulating cells from 666 cases were analyzed with a panel of markers by flow cytometry. The diseases included: chronic lymphocytic leukemia (CLL), 400; prolymphocytic leukemia, 22; hairy cell leukemia (HCL), 40; HCL variant, 15; splenic lymphoma with villous lymphocytes, 100; follicular lymphoma, 26; lymphoplasmacytic lymphoma, 25; mantle-cell lymphoma, 20; and large cell lymphoma, 18. On the basis of the most common marker profile in CLL, CD5+, CD23+, FMC7- and weak expression (+/-) of surface immunoglobulin (SmIg) and CD22, we devised a scoring system that gives for each of these five markers a value of 1 or 0 according to whether it is typical or atypical for CLL. Scores range from 5 (typical of CLL) to 0 (atypical for CLL). Application of the scoring system to all the cases showed that 87% of CLL scored 5 and 4 and only 0.4% scored 0 or 1, whereas 89% of other B-cell leukemias and 72% of lymphomas scored 0 or 1; only one case (0.3%) scored 4 and none scored 5 (p < 0.0001). There were no differences between CLL with high and low scores but higher scores were found in cases with more typical morphology (p < 0.0015). Considering each individual marker, there was no single one that distinguished CLL from other diseases, although the most reliable were SmIg intensity and FMC7. The proposed score will facilitate the diagnosis of B-lymphoproliferative disorders and improve their classification.