The present study consists of 1,238 women with unilateral breast cancer treated with modified radical mastectomy living in the geographic area of Haukeland Hospital. Their weight and height had been measured years before presentation of the disease. Age-adjusted Quetelet's index (weight/height2) showed that obese women had a 49% higher risk of dying from breast cancer than lean ones. The relative risk decreased slightly when adjusted for tumour diameter, lymph node status, and mean nuclear area of the tumour cells. The prognostic effect of Quetelet's index was examined according to the estrogen and/or progesterone receptor status of the tumour. In patients with a hormone receptor positive tumour, obese women had a risk that was more than three times higher than lean ones. In patients with hormone receptor negative tumour, the effect of obesity was reversed, lean patients having a risk that was more than six times higher than obese ones, even after adjustment for lymph node status, tumour diameter, and mean nuclear area. Quetelet's index, while being a prognostic variable in its own right, thus acts differently in patients with hormone receptor positive and negative tumours.