• To assess cellular immune function in children following severe blunt trauma, 25 children (mean: age, 7.1 years; Injury Severity Score, 34.9; and Glascow Coma Score, 5.6) admitted with severe trauma were examined with the use of the CMI Multitest system (Merieux Institute, Miami, Fla) to test delayed-type hypersensitivity. Patients were monitored for evidence of infection for the next 3 weeks. Ten children (mean: age, 6.2 years; Injury Severity Score, 31.2; and Glascow Coma Score, 5.4) admitted with severe trauma had the percentage of circulating lymphocyte subpopulations (pan—T cell marker T101, CD4, CD8, and B cells) measured on day 1 and then weekly for 3 weeks. Fourteen (56%; of the 25 children had no reaction to any of the skin tests (anergic). Eleven (79%) of 14 anergic patients became infected, while three (27%) of 11 of the nonanergic children became infected. There were no significant changes in pan—T cell marker T101, CD4, or CD8 lymphocyte populations in the 3 weeks following injury; however, absolute numbers of circulating B cells dropped significantly by day 7. These data indicate that children with severe trauma who are anergic are significantly more susceptible to infection. Unlike the results reported previously in adult trauma patients, these children had no significant fluctuations in T-cell populations; however, there was a significant decrease in circulating B cells in the first week. The use of the delayed-type hypersensitivity skin test can aid in identifying which patients are at an increased risk for nosocomial infection.