Background: Many reports indicate that the incidence and prevalence of diabetes mellitus is increased in schizophrenic patients and related to antipsychotic treatment. In an exploratory cross-sectional study we assessed the prevalence of type 2 diabetes mellitus in 266 chronic schizophrenic and schizoaffective inpatients and investigated whether the duration of antipsychotic treatment was related to the development of diabetes mellitus. Method: We measured the non-fasting plasma glucose level in 266 inpatients with DSM IV diagnosis of schizophrenia or schizoaffective disorder in 5 different long-stay wards in the Netherlands. Measured variables were: age, sex, ethnicity, BMI, current antipsychotic treatment, duration of illness and duration of antipsychotic treatment. Results: The overall prevalence of type 2 diabetes mellitus was 9%, which is significantly higher than the prevalence of 4.9% in the general population (OR 1.89, CI 1.14-3.13; p < 0.014). The prevalence was increased in two age cohorts: 30-39 years (3.8% vs. 0.3%, OR = 13.29, CI = 2.17-81.36, p = < 0.005) and 40-49 years (9.3% vs. 1.5%, OR = 6.74, CI = 2.77-16.38, p = 0.000). No new cases of diabetes mellitus were detected during the course of the study. The increased prevalence was found to be related to overweight and obesity. The time of exposure to antipsychotic treatment was not significantly correlated with the prevalence of diabetes mellitus when adjusted for age (F = 0.804, df = 1, p = 0.371, respectively, F = 0.194, df = 1, p = 0.660). Both typical and atypical antipsychotics contributed equally to the prevalence of diabetes mellitus. Conclusion: No significant relation between long-term antipsychotic treatment and prevalence of diabetes mellitus was found. The high prevalence of diabetes mellitus in schizophrenic patients warrants screening of these patients already at young age for glucose disturbance. © 2005 Elsevier B.V. and ECNP. All rights reserved.