OBJECTIVE: To describe the patterns and predictors of adherence with concomitant antihypertensive (AH) and lipid-lowering (LL) therapies. METHODS: This retrospective cohort study evaluated 8406 enrollees in a US managed care plan. Participants initiated both AH and LL therapies within a 90-day period. Adherence with AH and LL medications was measured at 3-month intervals from the start of concomitant therapy for up to 36 months (mean follow-up 12.9 months). Patients were considered "adherent" with AH and LL therapies if they had filled sufficient prescriptions to cover at least 80% of days with both classes of medications. A multivariate regression model evaluated potential predictors of adherence with concomitant therapy, including patient demographics, clinical characteristics, and health service use patterns at baseline. RESULTS: Six months after treatment initiation, only 36% of patients were adherent with both AH and LL therapies, 35% did not adhere to either regimen, and an additional 29% of patients were adherent with either AH or LL therapy, but not both. These proportions remained relatively steady over time. In the multivariate model, age, gender, time since treatment began, and a history of coronary heart disease or congestive heart failure were independently associated with the likelihood of being adherent. The number of other medications a patient was taking in the pretreatment year was strongly and inversely associated with adherence to concomitant therapy. In addition, patients were more likely to be adherent with concomitant therapy if they initiated AH and LL therapy on or about the same date (within 0-30 days of each other). CONCLUSIONS: Adherence with concomitant AH and LL therapy is poor, with only 1 in 3 patients adherent to both medications at 6 months. Initiating AH and LL therapy together and keeping the number of other medications to a minimum may improve adherence with concomitant therapy.