Concern has been expressed about the nutrition and health impact of high rates of soft drink consumption. Caffeine is an added ingredient in approximately 70% of soft drinks consumed in the United States. The soft drink manufacturers' justification to regulatory agencies and the public for adding caffeine to soft drinks is that caffeine is a flavoring agent.To examine the claim that caffeine plays an integral role in the flavor profile of soft drinks, by examining the effect of caffeine on the threshold for detection of flavor differences in cola beverages.Double-blind crossover study starting November 1998 and ending July 1999.An academic research center.Twenty-five adult regular consumers of cola soft drinks. Based on a screening session, all were able to detect a flavor difference between cola containing sugar and diet cola.A sensitive version of a forced-choice flavor-detection procedure was used to evaluate the effects of a wide range of caffeine concentrations (range, 0.05-1.6 mg/mL) on the ability to detect flavor differences between caffeinated and caffeine-free cola beverages. Repeated tests permitted determination of significant detection at each concentration in individual subjects.Percentage of subjects significantly detecting a flavor difference and mean percentage of trials correct at each caffeine concentration.Detection of flavor differences increased as a function of caffeine concentration. At the 0.1-mg/mL concentration, which is the approximate concentration in the majority of cola soft drink products, 2 subjects (8%) significantly detected a flavor difference and the mean percentage correct (53%) was at chance levels.The finding that only 8% of a group of regular cola soft drink consumers could detect the effect of the caffeine concentration found in most cola soft drinks is at variance with the claim made by soft drink manufacturers that caffeine is added to soft drinks because it plays an integral role in the flavor profile. It is valuable for the general public, the medical community, and regulatory agencies to recognize that the high rates of consumption of caffeinated soft drinks more likely reflect the mood-altering and physical dependence-producing effects of caffeine as a central nervous system-active drug than its subtle effects as a flavoring agent. Arch Fam Med. 2000;9:727-734