Although TREC expression drops off significantly with age, most patients exhibit an initial burst of thymopoietic activity that contributes to early lymphocyte recovery. (A) Total WBC count, percentage of lymphocytes, and expression of TRECs were assessed serially on all patients during the early leukocyte recovery period after timed sequential chemotherapy. The results from 1 patient who received AcDVP16 are shown. These results were representative of nearly all patients, regardless of chemotherapeutic regimen received. The majority of recovering leukocytes were lymphocytes. Although all patients had detectable TREC expression, 16 of 20 patients had a spike in TREC levels coinciding with initial lymphocyte recovery, suggesting that there is an initial burst of thymopoiesis that occurs during early lymphocyte recovery. (B) Age at start of chemotherapy had a significant negative predictive effect on both the median TREC expression (linear regression: r = 0.520, P = .019) and the maximum TREC expression (r = 0.624, P = .003) for a given patient. Median TREC expression was elevated in T lymphocytes derived from patients after chemotherapy compared with healthy controls. Healthy, age-matched persons had negligible TREC expression with a median of 47 copies/10⁵ lymphocytes compared with a median of 1873 TRECs/100 000 cells for patients (P = .015).