This article describes the development of a mobile arm support for people with muscular diseases. The arm support is spring-balanced, with special attention on reduction of operating effort (high balancing quality and low friction), functionality (large range of motion), and aesthetics (inconspicuous design). The spring settings can be adjusted for wearing heavier clothing or picking up an object, a function that can also be used for moving up or down. The device levels itself automatically to compensate for uneven floors, a function that can be overruled to assist forward/backward motion of the arm. Thus, the balancer can compensate for the weight of the arm and be adjusted to generate force to a limited (safe) extent. The principle and design of the mechanism are presented and preliminary field trial results are given. Two users report on 6 months of continuous use of the arm support in their home and social environments.