Cimetidine is known to have immunomodulatory effects and this study aimed to examine the effect of pre-operative cimetidine treatment on lymphocytic infiltration in n = 72 women with breast cancer randomised to 400 mg bd or placebo for five days presurgery. A combined index was devised by adding infiltrating lymphocyte percentage and lymphoid score. There were no significant differences in circumferential infiltrate and lymphoid follicles in cimetidine treated patients and control patients with breast cancer.