We have previously demonstrated that in patients with oestrogen deficiency, there was a significant reduction in the ratio of CD4+ to CD8+ lymphocytes. To test our hypothesis that oestrogen deficiency was the cause of the changes in lymphocyte subsets, we have studied the lymphocyte subsets and serum immunoglobulin levels in 35 patients with idiopathic premature ovarian failure before and after oestrogen replacement therapy. Before oestrogen replacement the lymphocyte subsets in peripheral blood were enumerated with indirect immunofluorescence techniques using monoclonal antibodies. The serum immunoglobulins (Ig) G, A and M were measured with nephelometry. After oestrogen replacement therapy, the tests were repeated around day 7-11 of the 2nd, 4th and 24th month. There was no significant change in any of these parameters by the 2nd or 4th months. After 2 years of oestrogen replacement, there was a significant decrease in the percentage of CD8+ cells, a significant increase in the CD4:CD8 ratio and a significant increase in the mean serum IgG concentration. There was no significant change in the other parameters. Our results support the hypothesis that oestrogen deficiency is the cause for the changes in the lymphocyte subsets.