Region-specific reference ranges for adult peripheral blood lymphocyte subsets have been established in few countries around the world, but most studies were restricted to younger adults for monitoring of HIV patients. The aim of this investigation was to establish age- and gender-specific reference ranges for healthy adults. Lymphocyte subsets were examined in 100 healthy volunteers (50 males, 50 females) aged 19-85 years by two-color flow cytometric analysis with a FACSCalibur. A statistically significant decline in the mean numbers of CD3+/CD8+ T cells and CD19+ B cells was observed beyond an age of 50 years, whereas the mean counts of NK cells and CD4+/CD8+ ratio significantly increased beyond the age of 50. Females < or = 50 years had significantly higher mean CD4+ T cell counts and lower NK cell counts than males < or = 50 years. Based on these results, we established reference values for three subgroups: males < or = 50 years, females < or = 50 years, and males/females > 50 years.