BACKGROUND: The European Union has banned sales of moist snuff (snus) in all member states, with the exception of Sweden. The ban is motivated by the potential adverse health effects of snus, but snus may also help people to avoid smoking or stop tobacco use. AIMS: The purpose of this study is to investigate the association between snus and smoking behaviour. MEASUREMENTS: The Swedish Survey of Living Conditions (ULF) health interview panel running from 1988/9 to 1996/7 was used to examine the gross and net flows between smoking and snus among Swedish males. Females were excluded from the analysis due to low snus prevalence. Contingency table models were used to investigate several hypotheses about the relationships between snus and smoking behaviour. FINDINGS: We found clear associations between the two habits. For the younger cohort (age 16-44 years), snus use contributed to approximately six smoking quitters per smoking starter attributable to snus. For the older cohort (age 45-84) there were slightly more than two quitters per starter. In terms of odds ratios, in the younger group smoking cessation attributable to snus was twice as common as smoking initiation, but in the older group the odds of starting smoking attributable to snus was 2.5 times higher than for quitting. CONCLUSIONS: Snus contributed to the reduction of smoking among Swedish males in the 1990s. Snus had different effects among non-smokers and smokers in different age groups.