Serious mental illness and substance abuse had little effect on offenses. However, substance abuse led to a sizable minority of offenses and was more likely than mental illness to cause an offense. Yet, unless it can be shown that factors unique to serious mental illness are specifically associated with behavior leading to arrest and incarceration, the criminalization hypothesis should be reconsidered in favor of more powerful risk factors for crime that are inherent in social settings occupied by persons with serious mental illness--risk factors such as unemployment, poverty, homelessness and substance abuse. With the belief that serious mental illness has been criminalized, this study investigated the effects of serious mental illness and substance abuse on criminal offenses. Participants of the study consisted of 113 community residents who participated in the Hawaii Jail Diversion Project and were arrested for a criminal offense. Tables, references