This brief paper reports on a study that used diagnosis, client self-reports, and clinician ratings to estimate the prevalence of drug, alcohol, and tobacco abuse among a random sample of 174 clients of 4 Assertive Community Treatment (ACT) teams in southeastern Ontario, Canada. Drug and alcohol abuse rates were lower that those reported in the literature, while high rates for tobacco use were consistent with published reports. ACT service delivery is guided by formal standards that assume high rates of substance abuse. It is argued that local population profiles should be monitored and considered in the development of ACT intervention practices.