AbstractCytologic evaluation of body fluids and fine needle aspirations (FNA) is frequently required in patients with hematologic diseases. In this study we have correlated immunophenotyping and DNA analysis by flow cytometry with cytologic findings and tissue biopsies from 20 patients with body fluid specimens and 5 with FNA. Nineteen of 25 cases, including all FNA cases, had an immunophenotype consistent with malignancy: 12 monoclonal lymphomas, 2 T‐cell lymphomas, 2 T‐ALL and 3 non‐T‐ALL. By cytologic examination, 14 of these 19 cases were positive for malignant cells, 2 suspicious and 3 negative; the latter 5 cases, including 2 FNA cases, had small monoclonal B‐cell populations detected by flow cytometry. Six cases had a benign immunophenotype; cytologic examination was benign in 4 of these and suspicious for lymphoma in 2. Our results show the feasibility of using flow cytometry to evaluate body fluids or FNA and demonstrate that small malignant populations that may be missed by routine cytology can be detected by flow