Mild lymphocytosis (< 10 x 10(9)/L) is a common finding in routine blood tests. When it persists, it raises the question of whether this disorder is an early manifestation of chronic lymphocytic leukemia (CLL). If it is accompanied by bone marrow infiltration, it can be safely considered as a sign of CLL. The aim of this study was to analyze retrospectively the usefulness of immunophenotyping and immunogenotyping for early detection of lymphocyte clonality in ambiguous cases of lymphocytosis without bone marrow infiltration. Twenty-six healthy individuals, 47 to 77 years old, with an absolute lymphocyte count (ALC) at the "onset" of the disorder between 4 x 10(9)/L and 9 x 10(9)/L, without marrow infiltration, were studied and followed for a period of 31 to 51 months. CD19, CD20, CD5, CD2, CD4, CD8 surface markers and amplification of the Ig heavy chain CDR-3 locus were used for immunophenotypic and genotypic analysis, respectively. Our studies indicate that immunophenotyping alone is sufficient and superior to CDR-3 locus amplification for the early detection of lymphocyte clonality in peripheral blood. Furthermore, the high frequency of CLL development in individuals with established monoclonality strongly suggests that patients with mild borderline lymphocytosis, even without bone marrow infiltration, have to be followed for progression to CLL and its possible complications.