Objective: Assertive community treatment (ACT) reduces hospitalizations for persons with severe mental illness. However, not everyone who needs ACT receives it. Without empirical guidelines for ACT planning, communities are likely to underestimate or overestimate the number of teams they need; thus the capacity of the programs will not meet current needs. In this study, administrative data were used to develop empirical estimates for the number of required ACT teams. These estimates were then used to examine current conceptual guidelines for developing the number of ACT teams that communities need. Methods: Administrative data from a large, urban county were used to enumerate all persons with a severe mental illness who had three or more hospitalizations within one year (ACT eligible). Results: Fifty-one percent of persons with a severe mental illness were found to be eligible for ACT (743 of 1,453 persons). This figure represents 2.2 percent of the county's mental health users and .06 percent of its adult population. Conclusions: Communities should develop enough ACT teams to serve approximately 50 percent of their populations of persons with severe mental illness or roughly .06 percent of their adult populations.