Aripiprazole is a novel antipsychotic with a mechanism of action different from those of traditional first- and second-generation antipsychotics. We describe three patients with long histories of treatment for schizophrenia or schizoaffective disorder in whom conversion to aripiprazole was being attempted. After they started aripiprazole, their psychosis, agitation, anxiety, or aggression worsened. Although the cause of the increased agitation was unclear, it may have been related to long-term use of dopamine-blocking antipsychotics and resultant upregulation of postsynaptic dopamine receptors. The mechanism of partial dopamine agonism observed with aripiprazole may increase dopaminergic activity and worsen positive dopamine-associated symptoms, such as paranoia, agitation, and aggression. The treatment of schizophrenia is often a clinical challenge, particularly when patients have a long history of noncompliance and poor response. Clinicians face difficult decisions in finding an effective and well-tolerated regimen. These cases magnify some of the challenges and provide insight into the clinical implications of converting to therapies with different pharmacodynamic effects.