Heavy episodic or "binge" drinking is commonly defined as drinking 4-5 drinks per occasion (5/4 definition) or drinking that results in a blood alcohol concentration (BAC) of .08%. The present study compared the validity of each binge definition as an indicator of at-risk, problem drinking. Two hundred and fifty-one college students were classified as nonbinge drinkers or as binge drinkers based on the 5/4 definition or the .08% BAC definition. The two definitions of binge drinking were examined in terms of their sensitivity and specificity as indicators of alcohol-related problems as determined by scores on the Alcohol Use Disorders Identification Test (AUDIT). Over half the sample (56%) were at-risk drinkers according to the AUDIT. The .08% definition detected only one-half of these individuals. Gender differences were also evident. Female binge drinkers actually achieved significantly higher estimated BACs per episode than their male binge drinking counterparts. The findings suggest that drinking to a subthreshold BAC (ie, <.08%) is not sufficient to avoid alcohol-related problems, and that total quantity (ie, total standard drinks) per occasion might contribute to risk independent of the BAC achieved during drinking episodes. The findings also highlight the importance of considering frequency of consumption in determining risky drinking versus relying solely on quantity measures.