The influence of dietary fat intake on subsequent change in body mass index (BMI) of adult women was examined while taking into account predisposition for obesity. A representative population sample of 361 Swedish women aged 38-60 y was first examined in 1968-1969 and followed up 6 y later. Dietary intake was estimated by diet history interview, and parental fatness was assessed by questionnaire. Women already overweight with > or = 1 obese parent were considered predisposed to obesity. When total energy intake, smoking habits, physical activity, and menopausal status were controlled for in regression analysis, high dietary fat intake was significantly associated with a 6-y gain in BMI only the predisposed women (P = 0.003), but not among obese women with lean parents, or lean women with or without obese parents. High dietary fat intake may have an obesity-promoting effect in women with a genetic predisposition.