From 1965 to 1990, the prevalence of cigarette smoking among US adults (aged > or = 18 years) fell steadily and substantially. Data for the 1990s suggest that the smoking initiation rate is increasing and that the decline in the prevalence of smoking may have stalled, raising the fear that the historical 25-year decline will not continue. The authors used a new dynamic forecasting model to show that although the decline may slow down, the demographics of smoking imply that prevalence will inexorably continue to decline over the next several decades, even without any intensified efforts aimed at tobacco control. The authors estimated and validated the model using historical (1965-1993) data collected by the National Health Interview Surveys on the prevalence of smoking among adults. Their results indicate that the current increase in the smoking initiation rate partially explains the fact that the prevalence of smoking has apparently leveled off, but even if the most grim assumptions about future initiation rates are used, the prevalence of smoking among adults will continue to decline for several more decades. The authors predict that if current initiation and cessation behaviors persist, the prevalence of smoking among adults will automatically decline from its current level of 25% to 15-16% by the second quarter of the next century. Even so, smoking will remain the nation's leading cause of premature death.