We retrospectively reviewed the clinical and laboratory data and bone marrow (BM) histomorphologic features in 33 angioimmunoblastic T-cell lymphoma (AITL) cases. Paraffin-embedded BM core biopsy specimens were reacted immunohistochemically with antibodies to pan-T-cell markers, CD20, CD10, CD21, and bcl-6. The frequency of BM involvement was 70% (23/33). The following features were more significant in patients with than without BM involvement: fever, hepatosplenomegaly, pleural effusion, elevated lactate dehydrogenase level, hypoalbuminemia, hyponatremia, hypocalcemia, anemia, circulating atypical cells, hypercellular marrow, and plasmacytosis in the BM. Morphologic features included nodular or interstitial infiltration in a paratrabecular distribution, periodic acid-Schiff-positive intercellular materials, mixed infiltrates of T and B cells, presence of clear cells, and blood vessel proliferation. Immunohistochemical assays revealed that CD5, bcl-6, and CD10 were useful markers of BM infiltration. Seven cases with BM involvement were interpreted as negative for lymphoma initially, mainly owing to insufficient information in nodal biopsy specimens. Several clinical and laboratory features indicate BM involvement of AITL at diagnosis. Laboratory physicians should be more familiar with BM histomorphologic and immunophenotypic characteristics suggesting AITL infiltration to make accurate diagnoses even without prior nodal biopsies.