We report 5 cases of intravascular lymphoma (IVL) initially diagnosed by bone marrow aspiration and biopsy. Each patient had generalized symptoms; 1 also had neurologic deficits. CBC counts revealed anemia (4 patients), thrombocytopenia (4 patients), or mild leukopenia (1 patient). The bone marrow biopsy specimen was diagnostic in each case. Lymphoma cells were present in small groups or single file in sinusoids (in 1 patient, sinusoids were distended markedly by IVL) and were detected in bone marrow aspirate smears (4 patients) and peripheral blood smears (all patients). Immunohistochemical studies demonstrated that every neoplasm was of B-cell lineage, CD20+, positive for other B-cell antigens, and CD3- or CD43-. Immunophenotypic studies revealed at least 2, and possibly 3, distinct immunophenotypic groups of B-cell IVL: CD20+ CD5+ (3 neoplasms), CD20+ CD5- CD10+ (1 neoplasm), and CD20+ CD5- CD10 unknown (1 neoplasm). B-cell IVL may be detected by morphologic examination of peripheral blood and bone marrow, and involvement of these sites may be more common than is reported in the literature. Immunophenotypic studies are helpful in establishing the diagnosis and suggest that B-cell IVL is a heterogeneous group of neoplasms that may arise from more than 1 normal B-cell precursor.