Nationally representative data on changes in 12-month prevalences of nicotine use, DSM-IV nicotine dependence, and DSM-IV nicotine dependence among users were analyzed to test the "hardening hypothesis," which proposes that declines in nicotine use resulting from population-level control measures leave a growing proportion of highly dependent users.Data were derived from two nationally representative surveys of U.S. adults: the 2001-2002 National Epidemiologic Survey on Alcohol and Related Conditions (NESARC, N=43,093) and the 2012-2013 NESARC-III (N=36,309). Weighted estimates of nicotine use, DSM-IV nicotine dependence, and an approximation of the Fagerström Test for Nicotine Dependence were compared for the 2001-2002 NESARC and 2012-2013 NESARC-III among the overall population and among nicotine users. Adjusted risk differences were obtained from logistic regression analyses using the predicted marginal approach.Between the 2001-2002 and 2012-2013 surveys, rates of 12-month nicotine use declined slightly (from 27.7% to 26.9%), but increased slightly but significantly when adjusted for sociodemographic characteristics (adjusted risk difference=1.4%). Larger significant increases were seen in 12-month nicotine dependence (adjusted risk difference=2.6%) and nicotine dependence among users (adjusted risk difference=6.4%). With few exceptions, increases in nicotine use, nicotine dependence, and nicotine dependence among users were statistically significant across most sociodemographic subgroups. Notable increases were seen among men; middle and older age groups; whites, blacks, and Hispanics; and the socioeconomically disadvantaged.Smaller increases in 12-month nicotine use relative to larger increases in 12-month nicotine dependence and nicotine dependence among users suggests that increases in nicotine dependence between the 2001-2002 and 2012-2013 surveys are findings that support the hardening hypothesis. Vulnerable subgroups of the population in terms of hardening were identified who would benefit from targeted nicotine dependence intervention programs to help them in overcoming dependence and quitting nicotine use.