Prompted by evidence from animal studies that castration produces lymphocytosis associated with thymic hyperplasia, this paper has reviewed serial lymphocyte levels in patients with prostate cancer undergoing hormone treatment. On day 7, 5 of 12 patients (58%) showed diminished circulating lymphocyte levels (compared to day 0) reflecting the surge of testosterone after treatment with LHRH analogues, while on day 28 a reversal had occurred and 13 of 17 (77%) had increased lymphocytes (compared to day 0). Possible relevance of these observations to defining a subgroup of patients with hormone-sensitive cancer who might benefit from immunotherapy is discussed.