Risk factors for breast cancer were compared in 107 women with interval breast cancer (cancers occurring within 2 years after a negative screen) and 258 women with breast cancer detected at 1st screening. All women (aged 40-67) were screened in the DOM project (the Utrecht programme for the early detection of breast cancer). Women with an interval cancer reported more often a history of benign breast disease (OR 4.66, 95% C.I. 2.08-10.41) and an artificial menopause (OR 4.07; 95% C.I. 1.74-9.55) than women with a screen detected cancer. Women with an interval cancer were taller than women with a screen detected cancer; this was seen most clearly in women with an artificial menopause (chi 2 for trend = 5.88; p = 0.02) and to a lesser extent in premenopausal women (chi 2 for trend = 1.70; p = 0.19). Premenopausal women with an interval cancer were heavier than women with a screen detected cancer (chi 2 for trend = 4.66; p = 0.03); whereas natural postmenopausal women with an interval cancer were leaner than women with a screen detected cancer (chi 2 for trend = 1.57; p = 0.21). All analyses were done while correcting for age and selected other risk factors for breast cancer. These results suggest that the epidemiological profile of pre- and post-menopausal women with an interval cancer differs from that of women with a screen detected cancer, which might imply a different natural history of these two types of breast tumours.