The study reports the performance of the Abbott CD3500 automated hematology analyzer for the enumeration and delineation of leukocyte populations for both adult and pediatric samples, and the ability of this instrument to detect the presence of abnormal cells. Samples from 542 individual patients either attending medical practitioners or during hospitalization were examined and then subdivided for the purposes of this study into 106 samples from newborn infants (< days), 145 samples from older children (15 days to 14 years) with non-oncologic disorders, 100 samples from normal adults, and 191 samples from oncology patient (97 adults and 94 children). The leukocyte differentials provided by both the Abbott CD3500 and the Coulter STKS were compared with those obtained from conventional morphology (two observers, total of 400 leukocytes). The sensitivities and specificities of the blast, immature granulocyte (IG) and NRBC "flags" were also determined. For the non-oncology adult (n=100) and pediatric (n=145) cohorts, automated differentials were given in all samples with the CD3500, whereas the STKS did not provide a differential analysis for 20 of the 145 (14%) pediatric samples, 11 of these were absent for no obvious reason. However, for the evaluable cases, the performances of the CD3500 and the STKS were broadly similar and generally correlated well with the manual reference procedure. The results for the newborn samples were less consistent with wider 95% confidence intervals (CI) noted. For example, the CD3500 (which reported a differential for all 106 samples studied) gave CI values of +/-14.4% for neutrophils, +/-14.6% for lymphocytes and +/-8.1% for monocytes. For comparison, the STKS (which did not provide a differential in 15% of 79 samples analyzed; insufficient material being available from the remaining 27 of 106 newborn samples) gave CI values of +/-21.9% for neutrophils, +/-23.5% for lymphocytes and +/-8.2% for monocytes. For all samples, the sensitivity of the blast flag on the CD3500 was 85% with a specificity of 91% (STKS: sensitivity, 75%; specificity, 85%); the sensitivity of the CD3500 IG flag was 72% with a specificity of 76% (STKS: sensitivity, 75%: specificity, 73%); and the sensitivity of the NRBC flag was 43% with a specificity of 94%(STKS sensitivity, 37%; specificity, 88%). This study confirms competitive performance levels for the CD3500 in the analysis of normal adult samples and suggests positive performance advantages in the study of neonatal, pediatric, and leukopenic samples.