The design and performance characteristics of ECAT-II, a second generation whole-body positron imaging system, providing high contrast, high resolution, quantitative images in two-dimensional and tomographic formats is presented. Discussed are: (i) a description of ECAT-II; (ii) the design criteria and implementation for maximizing image quality by minimizing effects such as random coincidence and scattered radiation, and by additionally performing dynamic background correction; and (iii) phantom studies to illustrate the implemented system resolution, efficiency, linearity, and field uniformity. Copyright © 1978 by The Institute of Electrical and Electronics Engineers, Inc.