CD3/CD20 immunostains are often performed in the initial cytological evaluation of lymphoid‐rich pleural effusions (LR‐PE). Most benign LR‐PE are predominantly composed of T(CD3+) cells while most malignant LR‐PE are of B(CD20+) cell lineage. As part of the effort to contain laboratory costs and improve diagnostic accuracy, there is increasing interest in applying principles of evidence‐based pathology to the use of immunostains. In this retrospective study, we reviewed the effectiveness of CD3/CD20 immunostains as a diagnostic or triage tool during the initial evaluation of 258 consecutive LR‐PE. 196 (76%) of the LR‐PE were ultimately diagnosed as reactive lymphocytosis and 62 (24%) as lymphoma/leukemia (L/L). There was a previous diagnosis of L/L, concurrent diagnostic tissue, and/or clinical evidence of L/L in 44 (71%) of the L/L effusions. An initial diagnosis of L/L was made in the remaining 18 (29%) cases. Sixteen of these 18 cases showed large cells with high‐grade features that mandated L/L workup. In only 2 (0.8%) of the 258 LR‐PE, CD3/CD20 stains were helpful to identify small cell lymphocytic lymphoma (SLL) in patients without concurrent peripheral lymphocytosis. CD3/CD20 immunostains do not appear to provide a cost‐effective method to diagnose or triage the vast majority of LR‐PE submitted to a clinical cytology laboratory. An algorithm that considers history, blood counts, and cytomorphology allows for cost‐effective selection of LR‐PE that warrant comprehensive hematopathologic workup. Our findings underscore the feasibility of applying evidence‐based principles to develop guidelines for the cost‐effective utilization of immunostains in cytology. Diagn. Cytopathol. 2012. © 2010 Wiley Periodicals, Inc.