Treatment guidelines for schizophrenia recommend that medical decisions should be shared between patients with schizophrenia and their physicians. Our goal was to determine why some patients want to participate in medical decision making and others do not. To identify determinants of participation preferences in schizophrenia patients (ICD-10 criteria) and in a nonpsychiatric comparison group (multiple sclerosis), we undertook a cross-sectional survey in 4 psychiatric and neurologic hospitals in Germany. Inpatients suffering from schizophrenia or multiple sclerosis (but not both) were consecutively recruited (2007-2008), and 203 patients participated in the study (101 with schizophrenia and 102 with multiple sclerosis). Predictors for patients' participation preferences were identified using a structural equation model. Patients' reports about their participation preferences in medical decisions can be predicted to a considerable extent (52% of the variance). For patients with schizophrenia, poor treatment satisfaction (P < .001), negative attitudes toward medication (P < .05), better perceived decision making skills (P < .001), and higher education (P < .01) were related to higher participation preferences. In the comparison group, drug attitudes (P < .05) and education (P < .05) were also shown to be related with participation preferences. Patients with schizophrenia who want to participate in decision making are often dissatisfied with care or are skeptical toward medication. Patients who judge their decisional capacity as poor or who are poorly educated prefer not to participate in decision making. Future implementation strategies for shared decision making must address how dissatisfied patients can be included in decision making and how patients who currently do not want to share decisions can be enabled, empowered, and motivated for shared decision making.