To the Editor: In their survey of diabetes mellitus in patients receiving neuroleptics, Michael J. Sernyak, M.D., et al. (1) reported that the prevalence was 9% higher in those treated with atypical neuroleptics than in those treated with typical neuroleptics. Dr. Sernyak et al. acknowledged some of the limitations of their study, including the fact that it was retrospective, there was no attempt to determine diabetes status at baseline, and the screening period was only 4 months long. They acknowledged that this narrow time frame yielded a virtual cross-sectional group, precluding determination of the temporal relationship between neuroleptics and the development of diabetes mellitus—a basic requirement when assessing causality. Thus, they showed an association between atypical neuroleptic treatment and diabetes, but they did not establish causality.