Volumetric capillary cytometry (VCC) is a new technology that involves the detection and enumeration of dually fluorochrome-labeled cells in a precise volume. We compared the accuracy and precision of VCC with the accuracy and precision of flow cytometry and hematology (F&H) for the measurement of the absolute numbers of CD4 and CD8 T cells in the whole blood of patients infected with human immunodeficiency virus. Five laboratories, each with a different F&H system and certified by the National institute of Allergy and Infectious Diseases flow cytometry proficiency testing program, were shipped aliquots of the same samples from a central site in addition to procuring samples locally. In general, the VCC technology generated CD4 and CD8 T-cell counts which were lower than those obtained with F&H. Intralaboratory variability of replicate CD4 T-cell determinations was similar for both technologies except in the local samples with CD4 counts less than 200/microliters, where the VCC variability was higher than the F&H variability. Interlaboratory variability on replicate CD4 T-cell counts made by VCC was significantly less than that when counts were made by F&H. The VCC instrument has automated CD4 and CD8 T-cell enumeration in whole blood and has consolidated the process to a single platform. Its performance in this evaluation indicates that it may represent a viable alternative to F&H for obtaining absolute T-cell subset counts.