Plasma folate quartiles by smoking and betelnut use among smokers, betelnut users, smokers and betelnut users, and control subjects. The subgroups in each folate quartile were as follows: quartile 1: 39% (n = 111), 30% (n = 85), 28% (n = 78), and 17% (n = 136); quartile 2: 27% (n = 75), 31% (n = 86), 25% (n = 70), and 22% (n = 179); quartile 3: 20% (n = 56), 23% (n = 65), 24% (n = 67), and 28% (n = 222); and quartile 4: 14% (n = 40), 16% (n = 45), 23% (n = 64), and 33% (n = 261), respectively. In the regression model that controlled for age and sex, the cigarette smoking × betelnut use interaction was significant (P = 0.04).