Obesity, measured by high body mass index (BMI >30 kg/m2) is associated with an increased risk of postmenopausal breast cancer but the effect of obesity on prognosis is not clear. A prospectively accrued and regularly validated database of 1579 patients with breast cancer treated in a district general hospital between 1963 and 1999 was analysed for clinical and pathological tumour characteristics including the family history, grade, tumour type, treatment and outcome. The risk factors and outcome of obese and non-obese patients were compared. Breast cancer in obese women was associated with significantly larger tumour size and worse Nottingham prognostic index. There was no statistically significant difference in overall and disease-free survival between obese and non-obese group. Hazard ratios (95% Cl) were 0.81 (0.62-1.06) and 0.80 (0.63-1.01), respectively. In the present study, obesity is not an indicator of worst prognosis of breast cancer.