We have previously shown that in patients with idiopathic premature ovarian failure there were significant changes in lymphocyte subsets. To test our hypothesis that these changes were due to oestrogen deficiency we studied lymphocyte subsets in patients with oestrogen deficiency due to other causes. Blood was taken for serum oestradiol, lymphocyte counts and lymphocyte subset counts (CD2+, CD4+, CD8+ and B cells) before oestrogen replacement in 19 patients with gonadal dysgenesis, 22 patients with hypothalamic-pituitary failure and 24 healthy female control subjects. The CD4:CD8 ratio in both groups of patients was significantly lower than that in the normal control subjects while the percentages and counts of lymphocytes and CD8+ cells were significantly higher. There was a significant positive correlation between the serum oestradiol level and the CD4:CD8 ratio. These findings support the hypothesis that the changes in lymphocyte subsets are due to oestrogen deficiency.