Abstract: Digital mammograms are automatically segmented into background, corresponding to the region external of the breast, and foreground, corresponding to the region within the breast, by the detection of the skinline which forms a border between these regions. A binary array is produced representing an initial coarse segmentation between foreground and background. The binary values are assigned in dependance on the sign of a spatial derivative approximating a gradient normal to the skinline. Spatial derivatives in fixed directions making a substantial angle with respect to the normal to the chest wall approximate the normal to the skinline direction in medial, lateral and lower contour sections of the breast, while a spatial derivative normal to the chest wall approximates the gradient normal to the skinline in anterior and upper contour sections of the breast.