AIMS: To measure changes over time in cigarette smoking uptake prevalence and timing during young adulthood (ages 19-26 years), and associations between time-invariant/-varying characteristics and uptake prevalence/timing. DESIGN: Discrete-time survival modeling of data collected from United States high school seniors (modal age 17/18) enrolled in successive graduating classes from 1976 to 2005 and participating in four follow-up surveys (to modal age 25/26). SETTING: The longitudinal component of the Monitoring the Future study. PARTICIPANTS: A total of 10 758 individuals reporting no life-time smoking when first surveyed as high school seniors. MEASUREMENTS: Smoking uptake (any, experimental, occasional and regular); socio-demographic variables; marital, college and work status; time spent socializing. FINDINGS: The percentage of young adults moving from non-smoker to experimental smoking [slope estimate 0.11, standard error (SE) = 0.04, P = 0.005] or occasional smoking (slope estimate 0.17, SE = 0.03, P < 0.001) increased significantly across graduating classes; the percentage moving from non-smoker to regular smoker remained stable. All forms of smoking uptake were most likely to occur at age 19/20, but uptake prevalence at older ages increased over time [e.g. cohort year predicting occasional uptake at modal age 25/26 adjusted hazard odds ratio (AHOR) = 1.05, P = 0.002]. Time-invariant/-varying characteristics had unique associations with the timing of various forms of smoking uptake (e.g. at modal age 21/22, currently attending college increased occasional uptake risk (AHOR = 2.11, P < 0.001) but decreased regular uptake risk (AHOR = 0.69, P = 0.026). CONCLUSIONS: Young adult occasional and experimental smoking uptake increased in the United States for non-smoking high school seniors graduating from 1976 to 2005. Smoking uptake for these cohorts remained most likely to occur at age 19/20, but prevalence of uptake at older ages increased.