Objective Given the limited evidence available about the effects of clean indoor air laws on smoking behaviour in the general population, the impact of national smoke-free workplace, bar and restaurant legislation, implemented on 1 January 2006, on smoking prevalence in Spain was assessed in this study.

Methods Population-based trend analysis using estimates for 27 periods from the beginning of 2000 to the end of 2008—three periods per year. To calculate the period per cent change in smoking prevalence, the permutation test for joinpoint regression to detect significant changes was used.

Results In men and women aged 15–24 years, the prevalence of smoking declined between the first period in 2000 and the third period in 2008, whereas in women aged 45–64 years, it increased by 1.7% per period. A declining trend was detected up to the first period in 2006 in men and women aged 25–44 years and in men aged 45–64 years, but between the beginning of 2006 and the end of 2008 the prevalence of smoking increased by 1.2%, 0.7% and 2.0% per period in men aged 25–44 years, in women aged 25–44 years and in men aged 45–64 years, respectively.

Conclusions 3 years after a national smoke-free law was implemented, the trend in smoking prevalence in some population groups was unchanged; however, in others, the declining trend of previous years was reversed. The similarity of these findings to those observed in other countries suggests that clean indoor air laws, although effective in reducing exposure to second-hand smoke, may not achieve the secondary objective of reducing the prevalence of smoking in the population.