We investigated patterns of cigarette smoking and Swedish snus (oral smokeless tobacco) use in a population-based sample of 19,073 Swedish twins 20-47 years old who participated in the baseline assessment of a prospective study of tobacco use and cessation in 2005-2006. Age-adjusted prevalence odds ratios (POR) and 95% confidence intervals (CI ) describe the association between tobacco use and sex, after adjustment for non-independence of twin pairs. Kaplan-Meier survival methods produced cumulative incidence curves of age at initiation of tobacco use. Slightly more than half of the baseline population was female (55.2%); the mean age at interview was 33.3 (+/-7.2) years and did not differ by sex. Having ever smoked daily was less common among males than females (11.9% vs. 15.3%; POR = 0.70 [0.64-0.77]), while having ever used snus daily was more common among males than females (31.1% vs. 4.8%; POR 11.7 [95% CI = 10.6-13.1]). The median age at initiation of smoking was 15 years for both sexes; median age at onset of snus use was 15 years for males and 18 years for females. Nicotine dependence scores were higher for males than females, and for current than former smokers. Findings from this study are in contrast to our previously published report on tobacco use among 32,123 Swedish twins 42-64 years old who completed a similar survey, and reported lower rates of snus use at later ages. Patterns of tobacco use may be changing in Sweden; snus use appears to be increasing, while daily smoking appears to be decreasing in popularity among the younger Swedish twins.