The incidence of red man syndrome (RMS) and its relationship to histamine were investigated in patients receiving vancomycin or an aminoglycoside (control). During the 60-min infusions, patients were observed for signs or symptoms consistent with RMS, including pruritus, erythema, angioedema, and cardiovascular depression. Four blood samples were obtained at 30-min intervals for determination of histamine concentrations. One (3.4%) of 29 vancomycin- and none of 8 aminoglycoside-treated patients had documented RMS. The mean maximum changes in blood pressure and heart rate were not significant and were similar between groups. Increases in histamine concentrations to > 1 ng/mL occurred only in 25% (2/8) of the aminoglycoside patients. Vancomycin induced minimal changes in histamine concentrations despite the occurrence of RMS. From these observations, it appears that RMS is not closely associated with histamine release, and elevated histamine concentrations do not predict RMS. Further investigation is needed to elucidate other mediators of RMS.