Sixteen fine needle aspiration (FNA) biopsies of lymphoblastic lymphoma (LBL) that were used to either initially diagnose disease (12) or document relapse (4) were reviewed. Cellular aspirates (2 x 10(7) cells) were readily obtained for immunologic, DNA/RNA flow cytometric and immunoglobulin and/or T-cell receptor gene rearrangement studies. Cytologically, aspirates were characterized by intermediate-sized cells (9.5-18.5 microns) with fine nuclear chromatin, small, inconspicuous nucleoli, irregular nuclear contours and scant basophilic cytoplasm. Frequent mitotic figures were seen (1-14 figures per 1,000 cells). Fourteen cases demonstrated a T-cell phenotype with considerable phenotypic variability. One case demonstrated a precursor B-cell phenotype, and another demonstrated biphenotypic expression with both T-cell and myeloid differentiation. Eleven of 14 cases (79%) were positive for terminal deoxynucleotidyl transferase. Thirteen of 15 cases (87%) manifested diploid DNA content by flow cytometric analysis and were characterized by intermediate proliferative activity (S+G2M 12.7 +/- 8.7% SD) and intermediate mean RNA index (1.3 +/- .5 SD). T beta gene rearrangements were demonstrated in four of five phenotypic T-cell LBL cases analyzed, with concomitant JH gene rearrangements observed in three cases, confirming that bigenotypic rearrangements characterize some T-cell LBLs. We conclude that FNA samples are adequate for accurate characterization of LBL and may obviate the need for surgical biopsy.