Snell-Bagg mice and Ames dwarf mice repeatedly show severe immunodeficiencies, affecting mostly the thymus- dependent lymphocyte system, probably caused by growth hormone deficiency. In growth hormone-deficient children contradictory data on the immune status have been reported. We investigated indices of cellular immunity in 22 adult patients with proven growth hormone deficiency in comparison to those in 100 health volunteers. Cellular immunity was assessed using total leukocyte count, percentage lymphocytes, and percentage and absolute numbers of CD3, CD4, CD8, CD19, and CD3-CD56+ (NK)-cells. Comparison revealed statistically significantly lower percentage and absolute number of NK-cells (P < 0.001). Except for a trend toward an increased CD4/CD8 ratio, no statistically significant differences for B- and T-lymphocytes could be observed. No correlation between the percentage and absolute number of NK-cells, on one hand, and the duration of growth hormone deficiency or prolactin level, on the other hand, could be demonstrated. In all these respects men did not differ from women. So, in growth hormone-deficient adults the percentage and absolute number of NK-cells are decreased.