Yun et al. BMC Public Health 2012, 12:724 http://www.biomedcentral.com/1471-2458/12/724 between the age of starting smoking and depression among current-smokers. We suggest that current smoking status is a more important factor for risk of depression. If a physiologic link indeed exists between smoking and the subsequent development of depression, then it would be reasonable to expect that evidence of this link would became more pronounced as the number of cigar- ettes consumed increases. Klungsøyr et al. reported a dose–response relationship with an increasing risk for former-smokers and for increasing numbers of cigarettes smoked per day for current-smokers, and also for an in- creasing number of smoking-years [18]. Pasco et al. reported that compared with nonsmokers, the odds for major depressive disorder increases by 1.47-fold for females who smoked 11–20 cigarettes per day and more than doubled for those who smoked >20 cigarettes per day [44]. However, in their studies, dose–response rela- tionships of smoking were evaluated by comparing them with never-smokers. Duncan and Reese reported very lit- tle evidence that smoking intensity was related to de- pression in smokers [45]. In their study, smoking an extra pack of cigarettes per month was associated with a statistically nonsignificant 0.02 increase in male CES-D scores and a 0.01 increase in female CES-D scores after controlling for smoking status [45]. In the present study, no dose–response relationship of smoking with depres- sion was observed among current-smokers in males, but female heavy smokers had a higher risk of depression. We suggest that this difference might be related to a gender-specific effect of smoking on depression. Conclusions Our data demonstrate that smoking is strongly asso- ciated with depression, particularly among females. However, a dose–response relationship was observed only between smoking and depression in females, and not in males, and duration of smoking cessation and age of starting smoking were not significantly associated with depression. Further large population-based pro- spective studies are needed to confirm the causal effects of smoking on depression. These findings suggest that depression prevention may need to be combined with smoking prevention and that different strategies in males and females may be necessary. Competing interests The authors declare that they have no competing interests. Authors' contributions WJY collected data, analyzed data and drafted the manuscript. MHS, SSK, SYR and JAR participated in the study design, data analysis and also reviewed the manuscript. All authors read and approved the final manuscript to be published. 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