Clonal immunoglobulin and T-cell receptor gene rearrangements are useful in distinguishing reactive lymphoproliferations from neoplastic processes. Here, we report a case of transient clonal expansion of CD8+ CD57- T-large granular lymphocytes (T-LGL) during primary cytomegalovirus infection. This case underlines that clonal expansion of T-LGL could be a reactive phenomenon related to an acute infectious disease and is not specific for lymphoid malignancy.