Absolute lymphocytosis after nonsurgical trauma was investigated in three patient groups at an acute-care tertiary referral hospital. The first group, with mild-to-moderate trauma, consisted of 64 patients who survived knife wounds to the chest and abdomen. Thirteen of the 64 patients had admission lymphocyte counts greater than 5.0 X 10(9)/L (mean +/- SD: 6.0 X 10(9) +/- 2.4 X 10(9]. Within 24 hours, all 13 showed a significant drop in lymphocyte count to 1.9 X 10(9) +/- 0.9 X 10(9)/L. The second group, with severe trauma, consisted of 11 patients admitted to the intensive care unit. Admission lymphocyte values averaged 5.9 X 10(9) +/- 0.6 X 10(9)/L and decreased to 1.54 X 10(9) +/- 0.3 X 10(9)/L within six hours. The relative importance of trauma as a cause of lymphocytosis was established by reviewing all hospitalized patients with lymphocyte counts greater than 5.0 X 10(9)/L between August 1983 and October 1985. The survey indicates that trauma and hemorrhage account for 16% of all cases of lymphocytosis, and that trauma, together with other acute stresses, constitutes the most common cause of lymphocytosis studied. The authors conclude that trauma is frequently associated with a lymphocytosis that usually changes to a lymphopenia within hours of injury.