The CD45RO phenotype of peripheral blood CD4+ and CD8+ lymphocytes were determined in physically active males (n = 171) and females (n = 70) ranging in age from 15-68 years. Direct immunofluorescence and dual color flow cytometry was performed for analysis. Absolute cell numbers (r = 0.473) and percentages of CD4+ CD45RO+ (r = 0.648) within the CD4+ population, percentage of CD8+ CD45RO+ cells within the CD8+ lymphocytes (r = 0.498) and total lymphocyte number (r = -0.242) correlated significantly (p < 0.001) with age. We conclude that the age-related increase of CD45RO+ T cells indicates a gradual increase of activated T cells.