O'Doherty and MacIntyre first drew attention to the strong link between smoking habits and palmoplantar pustular psoriasis in 1985.1 Other studies have suggested that smoking might be an important risk factor for chronic plaque psoriasis as well.*RF 2-5* Most of these studies have recorded information on smoking and psoriasis simultaneously but have also inquired into smoking habits before the onset of the skin disease. There seems little doubt that smoking and psoriasis are associated, but two critical questions remain unanswered: to what extent is the association a causal one, and how important are the implications for the public?

Though four out of six independent studies have suggested a twofold to threefold increased risk of psoriasis in smokers,*RF 2-7* these might all …