In many countries obesity has increased dramatically during the last decades, while there has been a parallel decrease in smoking. The objective of the present study was to estimate the net effect on premature mortality of these trends. A simulation model was developed to estimate the expected number of deaths between ages 19-56 years for cohorts of young men (n = 50,000), depending on inputs of obesity and smoking prevalence. The model was populated with nationwide data of Swedish men performing mandatory military conscription tests between 1969 and 2005. Risk equations for all cause mortality with smoking and obesity status as predictors were developed based on the 1969-1970 conscription cohort (n = 45,920; 2,897 deaths, median follow-up 38 years). It was found that between 1969 and 2005, the prevalence of smoking decreased from 58.6 to 23.2%, while overweight increased from 5.7 to 15.6% and obesity from 0.8 to 5.5%. As a result of these trends, a 14% (CI₉₅(%) 6, 21%) reduction of premature deaths between ages 19 and 56 years was forecasted for men aged 19 year in 2004-2005 compared to men aged 19 years in 1969-1970 (2,679 vs. 3,116 deaths). However, one-third of the survival benefit from reduced smoking during the period was offset due to the parallel increase in obesity. This study shows that despite large increases in overweight and obesity, a continued decline in premature deaths among Swedish males is expected due to reduced smoking during the last four decades.