The authors consider the optimum size of high-security units for mentally disordered offenders, taking into account principles of rehabilitation, quality of care, cost-effectiveness, probable length of stay, the architecture of secure units, and the psychology of dangerous persons. Although the authors conclude that an optimum absolute size for high-security units cannot be established, they present a formula based on nearness to and population density of the community where inmates are housed. A maximum-security hospital in Sydney, Australia, is described in terms of security, staff, costs, and inmates. An ideal high- security unit for mentally disordered offenders is also characterized. 1 reference