The objectives of this study were to determine the percentage and absolute counts of the peripheral blood lymphocyte subsets, and to examine the relationship between lymphocyte subsets and nutritional status, and total mortality in an institutionalised elderly population. Design The study had a cross-sectional and observational design. The sample of 115 permanent elderly residents was drawn from large geriatric institution in Melbourne, Australia. The main outcome measures were as follows: (i) percentages and absolute counts of lymphocyte subsets, (ii) association between biochemical indices of nutritional status (ferritin, iron and zinc) and peripheral blood lymphocyte subsets, (iii) total mortality during a 22-month period in relation to baseline lymphocyte subset counts. Women had higher absolute counts of various lymphocyte subsets than men. Positive correlations of serum ferritin with the number of CD8 (T-suppressor cell) and of serum iron with CD56 (natural killer, NK cells) were observed in men. In women, serum zinc was positively correlated with the absolute counts of CD3 (total T-cells), CD4 (T-helper cell) and CD19 (total B-cell). The analysis of survival data after 22 months showed that the mean number of CD4 cells of non-survivors (524 +/-292 x10(6)cells/L) was significantly lower than that of survivors (759+/-292 x 10(6) cells/L). The biochemical indicators of iron and zinc status partly account for variations in lymphocyte subset counts, consistent with known effects of iron overload and of zinc deficiency on immunocompetence. The number of CD4 T-cells may be useful in the prediction of total mortality in an institutionalised elderly population.