In addition to reducing breast cancer mortality, breast cancer screening programmes are expected to increase the proportion of patients who can undergo breast-conserving surgery. Trends in the use of breast-conserving surgery (BCS) in Southeast Netherlands between 1990 and 1998 were studied in relation to the gradual introduction of mammographical screening for women 50-69 years of age between 1992 and 1996. The characteristics of the tumours detected by the screening programme or outside of the programme were compared, to see whether this might clarify the observed trends. In the period 1990-1998, 4788 patients were diagnosed with invasive, operable breast cancer, of whom 2341 were 50-69 years of age. Although the screening programme resulted in a larger proportion of patients with small tumours and more favourable tumour characteristics, no increase was observed in the use of BCS for patients 50-69 years of age in the period 1990-1998 (64% in 1990 and 1998). Patients with a screening-detected tumour, however, were more likely to undergo breast conservation compared with those presenting clinically (68% versus 54%; P<0.0001). In conclusion, no increase in the proportion of breast-conserving surgical procedures was observed in Southeast Netherlands among patients 50-69 years of age in the period 1990-1998, during the introduction of mass mammographical screening for this group. Screening, however, resulted in a larger proportion of patients with small tumours with more favourable characteristics, who are better candidates for breast conservation.