Patients were not screened for preexisting diabetes mellitus, although it appears that this could have been done with available data. Identification and exclusion of preexisting cases would have avoided the possibility of assigning to specific neuroleptics cases of diabetes mellitus due to previous causes, including prior neuroleptic use. There is reason to believe that the likelihood of preexistence was not the same for all neuroleptics. For example, in a study on this subject by me and my colleagues (unpublished study by F.D. Gianfrancesco et al.), we found that 76% of the diabetes mellitus observed during treatment with quetiapine already existed within the 4 months before treatment, versus 70% for risperidone, 67% for olanzapine, and 71% for typical neuroleptics. Also, in 49% of the quetiapine-treated patients, treatment with quetiapine was immediately preceded by treatment with another neuroleptic, often olanzapine, whereas treatments with risperidone, olanzapine, and typical neuroleptics were immediately preceded by treatment with other neuroleptics in only 16%, 18%, and 13% of patients, respectively.