Mammographic breast density has been found to be a strong risk factor for breast cancer. In most studies it is assessed with a user assisted threshold method, which is time consuming and subjective. In this study we develop a breast density segmentation method that is fully automatic. The method is based on pixel classification in which different approaches known in literature to segment breast density are integrated and extended. In addition the method incorporates knowledge of a trained observer, by using segmentations obtained by the user assisted threshold method as training data. The method is trained and tested using 1300 digitised film mammographic images acquired with a variety of systems. Results show a high correspondence between the automated method and the user assisted threshold method. The Pearson's correlation coefficient between our method and the user assisted method is R = 0.911 for percent density and R = 0.895 for dense area, which is substantially higher than the best correlation found in literature (R = 0.70, R = 0.68). The AUC obtained when discriminating between fatty and dense pixels is 0.987. A combination of segmentation strategies outperforms the application of single segmentation techniques.