Objective To assess the adherence to a guideline for additional breast ultrasonography in a cross-sectional survey among hospitals in The Netherlands. Furthermore, consequences of current practice non-adherence for the patient outcome of diagnostic breast imaging were studied. Methods Current practice was compared with a guideline made up of three recommendations for the use of ultrasonography after mammography and three recommendations for not using ultrasonography. All patients referred for mammography to the radiology departments of the participating hospitals during 2 months in 2004 were eligible for the study. No data on the gold standard for breast cancer were analysed, but clinical consequences were estimated by using a probability model based on the data of a former prospective clinical study. Results In total, 17 of the 66 hospitals approached were participating in the study. Of the 13 694 patients assessed for eligibility, 6457 were included. High adherence rates (81-97%, mean 94%) were observed for the recommendations, which indicate additional ultrasonography, whereas lower adherence rates (68-94%, mean 83%) were seen for the recommendations which do not advise additional ultrasonography. Overall, in all included hospitals, non-adherence would result in 27.2 false-positive and 1.1 false-negative imaging results. Conclusion Current daily practice of diagnostic breast imaging in the hospitals in this survey corresponds to a great extent to the guideline proposed. Non-adherence in current practice results in a relatively small number of false-positive and false-negative imaging results.