The differential diagnosis for a neoplastic lymphocytosis based on morphology in the peripheral blood smear. The bolded diagnosis represents the image shown. Further recommended ancillary testing is listed in the third column. CLL, chronic lymphocytic leukemia; MBL, monoclonal B-cell lymphocytosis; MCL, mantle cell lymphoma; T-PLL, T-prolymphocytic leukemia; FL, follicular lymphoma; HCL, hairy-cell leukemia; SMZL, splenic marginal-zone lymphoma; HCLV, hairycell leukemia variant; LPL, lymphoplasmacytic lymphoma, T-LGL, T large granular lymphocytic leukemia; B-PLL, B-prolymphocytic leukemia; DLBCL, diffuse large B-cell lymphoma; ALCL, anaplastic large-cell lymphoma. Adapted with permission from Chabot-Richards D, George TI. Leukocytosis. Int J Lab Hematol 2014;36:279-88, page 282, Figure 2. *Pertussis infection is a reactive cause of monomorphic lymphocytosis.