Quantitation of morphological data is a component of both basic and clinical research. Often it is necessary to extract three-dimensional information from two-dimensional images. Stereological analysis, the most reliable means of achieving this, is also increasingly used as a diagnostic tool in pathology. COSAS, a COmputerized Stereological Analysis System, is software which allows rapid recording of morphological data from two-dimensional images and performance of analyses necessary for both quantitation and three-dimensional interpretation. COSAS is reliable and easy to use. It was previously available in a VAX-based version (Cornacchia and Black, 1988). An improved version, COSAS 2.0, is now available for use on the Macintosh. COSAS 2.0 is considerably more ''user friendly'' and flexible than the VAX version of the program. COSAS 2.0 eliminates the necessity for the user to be linked with a large computer. COSAS 2.0 has the ability to interface with other commercially available software for scanned images (e.g., Adobe Photoshop, NIH-Image) and data manipulation (e.g., EXCEL, Delta Graphics). These features make it useful to both research and clinical laboratories. Although designed for the biologist, the system may be used to study cross sections of any solid. It could be of interest to material scientists, petrographers, and ceramicists, all of whom need to obtain quantitative information about three-dimensional spatial organization from two-dimensional sections or projections, (C) 1996 Academic Press, Inc.