Experimental design: Cerebrospinal fluid (CSF) samples from 78 patients who received a diagnosis of epithelial-cell solid tumors and had clinical data suggestive of leptomeningeal carcinomatosis (LC) were studied. A novel FCI protocol was used to identify cells expressing the epithelial cell antigen EpCAM and their DNA content. Accompanying inflammatory cells were also described. FCI results (positive or negative for malignancy) were compared with those from CSF cytology and with the diagnosis established by the clinicians: patients with LC (n ¼ 49), without LC (n ¼ 26), and undetermined (n ¼ 3).Results: FCI described a wide range of EpCAM-positive cells with a hyperdiploid DNA content in the CSF of patients with LC. Compared with cytology, FCI showed higher sensitivity (75.5 vs 65.3) and negative predictive value (67.6 vs 60.5), and similar specificity (96.1 vs 100) and positive predictive value (97.4 vs 100). Concordance between cytology and FCI was high (Kp ¼ 0.83), although misdiagnosis of LC did not show differences between evaluating the CSF with 1 or 2 techniques (P ¼ .06). Receiveroperator characteristic curve analyses showed that lymphocytes and monocytes had a different distribution between patients with and without LC.Conclusion: FCI seems to be a promising new tool for improving the diagnostic examination of patients with suspicion of LC. Detection of epithelial cells with a higher DNA content is highly specific of LC, but evaluation of the nonepithelial cell compartment of the CSF might also be useful for supporting this diagnosis.Keywords: flow cytometry, immunophenotype, leptomeningeal carcinomatosis. L eptomeningeal carcinomatosis (LC) is a devastating cancer complication developing in at least 5% -10% of patients with solid tumors. Its prognosis is poor, with a median survival of 3-6 months among patients receiving chemotherapy.1 -3 Early diagnosis of LC and treatment initiation could offer the best chance of controlling symptoms and prevent the establishment of irreversible neurologic deficits that impair the patient's quality of life.3 -5 However, LC diagnosis currently remains a challenge. Cytological identification of malignant cells in the cerebrospinal fluid (CSF) remains the gold standard for LC diagnosis, but up to 45% of patients have an initial negative result of cytological examination of the CSF. This sensitivity may increase up to 90% when a high number of lumbar punctures are performed. 6,7 The implementation of imaging techniques and biochemical analysis of the CSF offers useful information for diagnosis; however, both of them should be considered in the right clinical context, because they lack specificity and their sensitivity is low. 1,3 It is therefore imperative to improve diagnostic tools.Multiparametric flow cytometry immunophenotyping (FCI) is an established and necessary laboratory instrument for diagnosis and follow-up of a wide range of hematological malignancies. In turn, FCI is not routinely used for the study of nonhematological tumors. Today, the number of monoclona...