Introduction. In recent years, e-cigarettes overtook cigarettes as the leading tobacco product used by U.S. adolescents. Most states, as well as federal regulations, have added e-cigarettes to laws prohibiting tobacco products sales to minors. We tested compliance with the newer regulation among Colorado urban retail businesses, speculating that violations might be more common for e-cigarettes than smokable cigarettes. Methods. Supervised minors visited a random sample of urban businesses and sequentially attempted to purchase an e-cigarette product and cigarettes. The protocol prescribed that the same minor make both attempts in each business, separated by at least a day to minimize influence of the first result on the second result. Data were collected during May 2014 - January 2015. Results. Among 238 businesses, more than one-fourth (26.1%; 95% confidence interval (CI), 18.9% to 33.2%) sold at least one type of product, and 6.3% (CI, 1.8% to 10.8%) sold both types. Violation rates were similar for e-cigarette products and cigarettes (17.6% vs. 14.7%, p=n.s.). Conclusion. Enforcement to prevent retail tobacco sales to adolescents should include e-cigarette products. Dual testing of stores indicates that single visits underestimate the problem of underage tobacco sales. IMPLICATIONS Adolescents can buy e-cigarettes from retail stores as easily as they buy cigarettes. Enforcement of tobacco sales laws should include e-cigarettes, and test-retest protocols are needed to estimate the true extent of the problem.