Body height and weight in relation to breast cancer in women younger than 45 years were investigated in a case-control study in Sweden and Norway. The study included 317 Swedish and 105 Norwegian cases diagnosed in 1984-85 with 317 Swedish and 210 Norwegian age-matched population controls. Neither height nor body size, measured as body mass index, was associated with breast cancer. Change in body mass from the age of 20 years to 18 months before the time of diagnosis (cases) or interview (controls) had no effect on breast cancer risk. The study provides no evidence that anthropometric measures are risk factors for breast cancer in young women, indicating that the postulated inverse relationship between body mass index and pre-menopausal breast cancer could be limited to peri-menopausal women.