The Community Health Center Network measured the prevalence of glycemic control in diabetic patients at 7 community health centers as part of its clinical quality improvement program. A cross-sectional survey was carried out in a random sample of 1817 diabetic patients having 1 or more encounters from October 1, 2000 to September 30, 2001. Computerized laboratory results for hemoglobin A1c (HbA1c) tests were available for half the sample. Manual review of medical charts was carried out for the rest. The proportion of diabetic patients with 1 or more HbA1c tests in the measurement year was 91% (CI95%: 90-93%) and poor glycemic control (HbA1c > 9%) occurred in 27% (CIM%: 25-30%). The mean of the most recent test was 7.8%. The frequency of testing varied significantly by clinic from 79% to 94% and increased with the number of encounters. Poor glycemic control also varied significantly by clinic (17-48%) and was significantly better in females and older patients. Measures of glycemic control were not associated with ethnicity or insurance status in multivariate analyses. A high proportion of diabetic patients received appropriate care, and this care was not associated with ethnicity or insurance status. The data warehouse was an essential tool for the clinical quality improvement program.