The risk for major depression was 27% in heavy smokers, compared to 7% in patients who previously smoked but had quit. Heavy smokers were defined as those who smoke a pack a day or more. It was not clear if smoking caused the depression or if patients genetically prone to depression were also prone to smoking, but the association added one more risk factor to this deadly habit.
See Salma Khaled at the University of Calgary, J. Psychiatric Research 2012;46:436-43.