Analysis of 1684 inpatient and 884 outpatient peripheral blood counts revealed the following: (1) The absolute lymphocyte count declines sharply from 5000/μ,l to 2000/μl in the first two decades, remains constant for three decades, then declines at an accelerated rate beginning in the 40s, to reach 1500/μl at age 90. (2) The absolute granubcyte count does not show age-dependent variation, remaining essentially constant throughout life. (3) The values of absolute lymphocytes are indistinguishable for ambulatory or hospitalized subjects except for higher absolute lymphocyte counts in hospitalized children in the first decade. (4) The data support other evidence for declining cellular and humoral immunity in aging man.