Currently, CD4+ lymphocyte counts are one of the most widely used surrogate markers for monitoring disease progression in and initiating therapy for human immunodeficiency virus-infected individuals. However, the process of obtaining lymphocyte subset counts can be complex and expensive, often rendering the test inaccessible to many patients. In contrast to standard laser-based flow cytometry, the TRAx CD4 Test Kit utilizes an enzyme-linked immunoassay format to provide CD4+ lymphocyte counts by a simple and more cost-effective means. In order to evaluate the utility of the TRAx CD4+ assay in comparison with flow cytometry, heparinized blood samples were drawn from 188 infected and uninfected adult patients and 24 infected pediatric patients and evaluated by both assays. The correlation coefficient for all adult individuals tested was 0.94, and the mean absolute counts (in cells per milliliter, +/- standard deviation) were 510 +/- 358 for TRAx and 480 +/- 361 for flow cytometry. The correlation for the pediatric group was 0.93, with mean absolute counts of 956 +/- 767 for TRAx and 1,521 +/-a 1,438 for flow cytometry. Overall the TRAx CD4 Test Kit performed well in comparison to flow cytometry, and its lower cost and ease of use make it an encouraging alternative for the routine determination of CD4+ lymphocyte counts.