A long-standing challenge of content-based image retrieval (CBIR) systems is the definition of a suitable distance function to measure the similarity between images in an application context which complies with the human perception of similarity. In this paper, we present a new family of distance functions, called attribute concurrence influence distances (AID), which serve to retrieve images by similarity. These distances address an important aspect of the psychophysical notion of similarity in comparisons of images: the effect of concurrent variations in the values of different image attributes. The AID functions allow for comparisons of feature vectors by choosing one of two parameterized expressions: one targeting weak attribute concurrence influence and the other for strong concurrence influence. This paper presents the mathematical definition and implementation of the AID family for a two-dimensional feature space and its extension to any dimension. The composition of the AID family with L p distance family is considered to propose a procedure to determine the best distance for a specific application. Experimental results involving several sets of medical images demonstrate that, taking as reference the perception of the specialist in the field (radiologist), the AID functions perform better than the general distance functions commonly used in CBIR.KEY WORDS: Distance function, medical images, content-based image retrieval INTRODUCTION D igital images, which are present in the majority of medical systems, serve to support diagnostic activities. However, for the effective and suitable use of images, these systems must include tools for image management, including fast and effective comparison and retrieval. In the image analysis environment, the ability to compare images automatically is important because the number of stored images is usually very large, precluding a radiologist from making individual comparisons of the images in the entire database. Some groups of radiologists prefer to search in reference libraries, which are composed of sets of typical images. However, the possibility of retrieving images containing patient-related information from specific databases enables the analyst to make more in-depth analyses, for instance, to explore and study the patients from a particular geographic region.The core of content-based image retrieval (CBIR) is the use of intrinsic visual features, which are extracted automatically from the images to describe them while keeping their most relevant characteristics. Such features, usually consisting of numerical values obtained by image-processing algorithms, are used to compare and index images and are usually placed together in a feature vector. Each item of a feature vector is also called an image attribute. CBIR techniques take advantage of index structures that use the similarity of features to speed up the retrieval, doing this automatically. 1,2 In Figure 1, the user places a query, which is done typically by providing the system with an image (query image) ...