A comparison of flow cytometry (FC) and bone marrow biopsy (BMB) to evaluate bone marrow infiltration was made in 114 patients suffering from B-cell non-Hodgkin's lymphomas (NHLs; 51 at diagnosis, 63 during post-therapy follow-up). The following parameters were indicative of bone marrow infiltration: altered surface k/l ratio; specific immunophenotypic pattern in particular NHLs (CLL, mantle cell lymphoma, hairy cell leukemia). FC and BMB agreed in 89.5% of cases (i.e. both showed 48 positive and 54 negative cases). In discordant cases (7.9%) and in cases not evaluable by FC (2.6%) IgH rearrangement and bcl-1 gene expression, both evaluated by PCR methods, were used to detect bone marrow infiltration with higher precision. These results show that a more complex analysis of bone marrow is needed to diagnose bone marrow infiltration, particularly in samples with minimal residual disease.