OBJECTIVE: To examine prevalence and co-occurrence of diabetes mellitus (DM), hypertension (HT), and elevated low-density lipoprotein cholesterol (dyslipidemia, or DL) in a managed care population. STUDY DESIGN: Period prevalence study. PATIENTS AND METHODS: The study population included all adults (age > 20 years) who had been members of Kaiser Permanente, Northern California, for at least 4 months on December 31, 2001 (n = 2.1 million). Criteria from the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of Hypertension, the Third Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults, and the Northern California Kaiser Permanente Diabetes Registry were applied to computerized databases for an 18-month period to identify HT, DL, and DM, respectively. Because screening for these conditions is incomplete, we applied age- and sex-specific prevalence estimates from the Third National Health and Nutrition Examination Survey to simulate full ascertainment. RESULTS: Unadjusted prevalence rates of HT, DL, and DM were 23.8%, 17.6%, and 6.6%, respectively. More than 50% of persons with either HT or DL also had at least 1 other condition. Of all persons with DM, 74% had HT, 73% had DL, and 56% had both. Under full ascertainment, prevalence increased to 27.6%, 35.6%, and 8.7% for HT, DL, and DM, respectively, and co-occurrence increased further. CONCLUSION: HT, DL, and DM co-occur in most affected individuals. To avoid fragmentation of care, disease management strategies should aim to manage these conditions within the same programs.