Beliefs about the causes of schizophrenia are thought to impact treatment outcomes. We investigated 3 theoretically opposing belief systems (biological, psychosocial, magical-religious) in relation to the severity of positive and negative symptoms and to attitudes towards medications. We recruited 253 patients with schizophrenia and their primary caregivers from public mental health clinics in Bolivia, Chile, and Peru. We assessed patients’ and caregivers’ beliefs about the causes of schizophrenia, which were used as predictors of symptom severity and medication attitudes in linear regression analyses. Patients’ scores on biological, psychosocial, and magical-religious beliefs were positively correlated with one another, indicating that these domains were not, as anticipated, “opposing.” Patients with higher levels of biological and psychosocial beliefs had significantly lower levels of positive and negative symptoms; in contrast, higher levels of magical-religious beliefs were associated with increased positive symptoms and less favorable attitudes towards medications. Patients’ belief systems are significant predictors of symptom severity and medication attitudes. Research is needed on the extent to which psychotherapeutic treatments for schizophrenia should bolster patients’ beliefs in the biological and psychosocial domains and weaken beliefs in the magical-religious domain; this research should also attend to the ethical considerations involved in intervening on belief systems cross-culturally.