Monoclonal B cell lymphocytosis (MBL) is both a marker of immune senescence and a potential precursor of B cell malignancy. Most MBL populations have a chronic lymphocytic leukemia-like (CLL-like) immunophenotype, but those that are CD5-negative (non-CLL-like) are also recognized and may represent a distinct diagnostic entity. To date, MBL studies have taken place in relatively homogenous populations, although risk of CLL varies across racial groups and geographic regions. We report flow cytometry data from 597 ethnically diverse 64-94-year-old women from across the USA who are participants in the Women's Health Initiative (WHI) Long-Life Study (LLS). Overall, MBL was detected in 26 % of the participants and included 20.9 % with a CLL-like immunophenotype, 5 % with a non-CLL-like immunophenotype, and 1.3 % with both. White and Hispanic women were more than twice as likely to have a CLL-like MBL population detected than African American women, corrected for age (P = 0.003). By contrast, detection of non-CLL-like MBL did not vary significantly by race, but did increase markedly with advancing age, being present in 12.7 % of those aged 85 and older. We provide new evidence that rates of detection of CLL-like MBL are lower in African Americans, and further suggest that non-CLL-like clonal expansions should be regarded as distinct from CLL-like MBL.