Cerebrospinal fluid (CSF) involvement in B cell non-Hodgkin lymphomas is a poor prognostic sign and diagnosis is made using techniques such as flow cytometry (FCM) and conventional cytology (CC). Aim: To evaluate the frequency of CSF involvement in B-NHL by both techniques in a public hospital. Material and methods: 97 CSF samples were analyzed in tubes with cell preservative belonging to 70 patients, 71% male, median age 56 years (18-85 years), with a diagnosis of B-NHL and risk of infiltration according to medical criteria. Most were patients from new diagnosis (89%), diffuse large B cell lymphoma (60%), and Ann-Arbor stage III-IV (77%). In 67 samples (69%), CC and CMF were performed simultaneously. Results: Of the samples analyzed by CMF, 99% were valuable, while by CC, only 67% (p<0,05). Globally, 25% of the samples showed infiltration by CMF, while 18% by CC (p<0,0001). Forty-four valuable samples were evaluable and analyzed by CC and CMF, finding a similar frequency of positive cases (27%), with two-thirds positive only by CC or CMF. Positive samples in diffuse large B cell lymphoma were 28% by CC and/or CMF. Conclusions: A higher proportion of infiltration cases were detected by CMF than by CC. In valuable cases, CC complements CMF.