A thorough analysis of the immunological status was conducted in a 15-month-old child with progeria (Hutchinson-Gilford syndrome). Total leukocyte and neutrophil counts were slightly increased, and monocytes were decreased. Percentage and number of CD4+ cells in the blood were mildly decreased as well as the CD4/CD8 cell ratio. CD20 (B-cell marker) bearing cells and cells bearing Ia-antigens were increased, as well as CD16 and CD56 marker-bearing cells (natural-killer cells, NK). Kymphocyte proliferation upon stimulation with phytohaemagglutinin and purified protein derivative were decreased, and with pokeweed mitogen increased. NK cell activity appeared increased, particularly at lower effector: target cell ratios.