We investigated early immunological reconstitution and the production of circulating inflammatory mediators and their relationship to aGVHD in children during the first 100 days following unrelated UCBT. Nine patients had an underlying malignant disease (ALL, ANLL), and two, non-malignant diseases (SAA, ALD). The median age was 10 years (range: 1.25-21). Seven of 11 patients were alive by day 100, two died from regimen-related toxicity, and two died from severe aGVHD (grade >/=III). Myeloid engraftment (ANC >/=500/mm3 x 2 days) occurred at a median of 24 days (range: 14-55), while platelet engraftment (platelet count >/=20 000/mm3 untransfused x 7 days) was delayed and occurred at a median of 52 days (range: 33-95). The mean cell dose of CD34+ cells was 3.3 +/- 3.51 x 10(5)/kg, and of CD34+/CD41+ cells was 3.94 +/- 3.99 x 10(4)/kg. Acute GVHD (grade II-IV) developed in seven patients (77%), and severe aGVHD (grade III-IV) developed in five patients (55%). Serum levels of IL-2Ralpha, IL-2, IL-4, IL-7, IL-12, and IFNgamma were not significantly different between patients with grades 0-I aGVHD and patients with grades II-IV aGVHD. Evaluation of immunological reconstitution on day 90 post UCBT demonstrated an early recovery of the absolute numbers of B cells (CD19+) and NK cells (CD3-/CD56+). Immunoglobulin levels for IgG, IgM and IgA remained normal throughout the study period. PMN functional studies demonstrated normal superoxide generation, bacterial killing (BK), and chemotaxis (CTX). However, both helper (CD3+/CD4+) and suppressor (CD3+/CD8+) T cell subsets remained low during the first 100 days post UCBT with mean +/- s.e.m. values of 120 +/- 29/mm3 and 10 +/- 50/mm3, respectively (normal = 900-2860/mm3 (CD3/CD4), normal = 630-1910/mm3 (CD3/CD8)). Mitogen response studies showed low blastogenesis to PHA and PWM, with a mean c.p.m. +/- s.e.m. value of 1.7 +/- 0.67 x 10(4) for PHA (NL >/= 75 x 10(3)) and 8.42 +/- 4.1 x 10(3) for PWM (NL >/=25 x 10(3)). In conclusion, serum levels of inflammatory mediators were not predictive nor did they correlate with the severity of aGVHD. Recovery of NK cells, B cells, and PMN functions occurred within the first 90 days post transplant. However, T cell subsets, CD3+/CD4+ and CD3+/CD8+, and T cell functional activity remained significantly decreased and may account for the high incidence of infectious morbidity seen during this immediate post UCBT period.