Several caveats deserve mention. First, we used ICD-9 codes to identify MHCs. While this is a common approach in mental health services research,2,17,50,51 it underdetects MHCs because of underdiagnosis52- 54 or underdocumentation.55 Therefore, our no-MHC group undoubtedly included some MHC cases, making the 2 groups more similar and diluting the strength of observed effects. However, the stability of findings using high-specificity and high-sensitivity definitions is reassuring. Second, while we adjusted for known confounders (eg, sex56,57 and medical comorbidity58,59), we cannot exclude the possibility of unmeasured confounders explaining differences of this magnitude. Third are issues related to data capture. Only patients whose primary VHA facility was one of the 78% submitting complete laboratory data to central VHA files were included. This is unlikely to have introduced important bias, since diabetic patients at included facilities resembled diabetic patients at excluded facilities on demographic characteristics. Although our focus was on differences in diabetes measures for patients with vs without MHCs (rather than absolute rates), it is reassuring that the overall rate of adherence that we observed was in line with rates observed in contemporaneous VHA and non-VHA studies. For example, in our study, lack of HbA1c testing, lack of lipid testing, and lack of eye examination were seen in 17%, 30%, and 41% of VHA diabetic patients, respectively, in 1999. In the same year, rates were 7%, 27%, and 33%, respectively, in one VHA study13 and 6%, 29%, and 27%, respectively, in another VHA study.60 (The somewhat better rates of testing in those studies compared with ours likely reflect the fact that their samples included regular users of primary care; because of concerns that patients with MHCs might access primary care differently, we included patients who were not enrolled in primary care so as to avoid selection bias.) In a national study where medical records were abstracted for Medicare patients nationally in 1998 to 1999, the corresponding rates were 30%, 26%, and 32%, respectively.12 Finally, the VHA health care system emphasizes care of patients with MHCs. It is likely that disparities may be larger in systems with less integrated medical and mental health care. To test generalizability, it will be important to replicate our findings in diverse clinical settings.