Lymphocytosis, defined by an increase in absolute lymphocyte count (ALC) to more than 4000 lymphocytes/microL in adult patients, is a common hematologic abnormality. ALC calculates as the total white blood cell count (WBC) multiplied by the percentage of lymphocytes in the peripheral blood. Different lymphocyte subsets (T cells, B cells, or NK cells) may be increased depending on the particular etiology. Lymphocytes represent around 20 to 40% of WBC. The definition of relative lymphocytosis is an increase in WBC of more than 40% in the presence of a normal absolute white cell count. In this review, we present the most common causes in adult patients, in addition to a general approach to diagnosis and management of frequently encountered etiologies.