Flow cytometer (FACScan) was used to determine the range of T lymphocyte subpopulations in normal Thai blood donors at Maharaj Nakorn Chiang Mai Hospital, Chiang Mai. Reference population consisted of 150 healthy HIV seronegative blood donors. T lymphocyte subsets were analysed using two-color immunophenotyping of peripheral blood lymphocytes with a lysed whole blood technique and enumerated. The study showed that the normal values for CD3+ lymphocytes (percent), CD4+ lymphocytes (percent), CD8+ lymphocytes (percent), CD4/CD8 ratio, absolute CD3+ lymphocyte count, absolute CD4+ lymphocyte count and absolute CD8+ lymphocyte count were 64 +/- 8.8, 36.1 +/- 6.4, 25.7 +/- 7.3, 1.5 +/- 0.6, 1,630 +/- 600 cells/microl, 910 +/- 300 cells/microl and 670 +/- 350 cells/microl, respectively. We found that the values of CD3, CD4 and CD4/CD8 ratio were significantly lower than those in the Caucasians but those of CD8 was not significantly different. This observations have important clinical implication for the use of T lymphocyte subsets measurement, especially in the management of HIV infection in Thais. These normal ranges can be used as a reference for the decisions in clinical practice.