Cimetidine has demonstrated a survival benefit in a randomized trial as adjuvant therapy for gastric cancer. We have demonstrated expression of histamine receptors on colon cancer cell lines and inhibition of their growth with cimetidine. Cimetidine also activates suppressor T cells and stimulates cell-mediated immunity. We therefore performed a randomized controlled clinical trial to determine the effect of cimetidine 400 mg given twice daily in conjunction with chemotherapy vs chemotherapy alone. Thirty-eight patients were randomized and 35 patients were eligible for further analysis. Both groups were well matched for pre-treatment characteristics. There was no difference in overall response. There was, however, a significantly increased rate of CEA response in the cimetidine group. Four of 11 patients (36%) in the cimetidine group had a CEA response compared to.none of eight in the control. Meaningful comparisons of overall survival cannot yet be made. This study demonstrates that cimetidine has encouraging activity in increasing CEA response in patients with metastatic colorectal cancer treated with chemotherapy. This observation needs to be extended in a larger randomized study, which is currently underway. © 1995 W.B. Saunders Company Limited.