AbstractPurpose To examine short-term consequences associated with consuming alcohol and energy drinks compared with consuming alcohol without energy drinks. Methods A longitudinal measurement-burst design (14-day bursts of daily surveys in four consecutive college semesters) captured both within-person variation across occasions and between-person differences across individuals. The analytic sample of late adolescent alcohol users included 4,203 days with alcohol use across up to four semesters per person from 508 college students. Results Adding energy drink use to a given day with alcohol use was associated with an increase in number of alcoholic drinks, a trend toward more hours spent drinking, elevated estimated blood alcohol content (eBAC), a greater likelihood of subjective intoxication, and more negative consequences of drinking that day. After controlling for eBAC, energy drink use no longer predicted subjective intoxication but was still associated with a greater number of negative consequences. Conclusions The consumption of energy drinks may lead to increases in alcohol consumption and, after controlling for eBAC, negative consequences. Use of energy drinks plus alcohol represents an emerging threat to public health.