A higher prevalence of diabetes mellitus (DM) has been reported in schizophrenic patients for decades. But the risk factors for diabetes in these patients were not well understood. The aim of our study is to establish the prevalence of DM in patients with schizophrenia in Taiwan and discover the risk factors associated with it. In 246 recruited schizophrenic patients, the prevalence of DM was 9.8%, not significantly different from that of the general population in Taiwan. However, we found a significantly higher prevalence of DM in younger schizophrenic patients (p=0.001) than in the general population, but not in older patients (p>0.10). Our results showed old age, a longer duration of illness, obesity, and having a family history of DM were independent factors associated with DM in schizophrenia. These results suggest that schizophrenia may make young people more vulnerable to developing DM. Further large-scale, controlled studies should be done to confirm our results.