Surgical trauma triggers a significant, but transient, fall in blood lymphocytes in the immediate post-operative period. The relevance of this fall to the controversial issue of post-operative immune suppression is unclear. In order to evaluate whether any long-lasting changes in T-lymphocyte subset distribution occur following trauma, lymphocyte populations and sub-populations were estimated using anti-Ig and monoclonal antibodies in a group of patients undergoing routine elective surgery and in a group of age- and sex-matched normal controls. Surgery triggered an approximately equal percentage fall at 24 hr in both the T-helper and T-suppressor cell populations, so that the Th:Ts ratio remained constant and identical to the control value. Th-cell numbers tended to recover more quickly than Ts cells. The ratio either remained normal or tended to increase by the 7th post-operative day. There was no evidence to suggest that alterations in the distribution of lymphocyte subsets contribute to post-operative immune suppression in the vast majority of surgical patients. Only 2 out of 17 patients developed decreased ratios. The significance of these findings is discussed.