OBJECTIVES: Market concentration of products affects competition, and thus is a key concern of marketing firms, policy makers, and regulators. Angiotensin converting enzyme inhibitors (ACE inhibitors) and angiotensin receptor blockers(ARBs) are two classes of hypertensive drugs, competing for one of the largest and most profitable pharmaceutical market in the US. However, little is known about how market concentration affects the consumption of these drugs. We sought to measure the concentration of drugs in ACE inhibitors and ARBs and its association with their relative market share.  METHODS: We used the State Drug Use Data from the Medicaid Drug Rebate Program that provides prescription drugs to 46 million low-income Americans. We linked the data with the WHO Collaborating Centre for Drug Statistics Methodology to obtain Defined Daily Dose (DDD) measurements for drug consumption for four large states,   for a total of 16 quarters of continuous measurements between 2005 and 2008. We used a Herfindahl-Hirschman Index (HHI) to measure the concentration in ACE inhibitors and ARBs, by DDD and reimbursements. We conducted multivariate GLS regression analysis by regressing the HHI on the relative market share of ACE inhibitors, adjusted for time trend, the interaction between the HHI and time trend, and random effects of states. RESULTS: During the 16 quarters between 2005 and 2008, the mean concentration index for ACE Inhibitors was 339 (s.d. 71) by DDD, 442 (s.d. 148) by reimbursements, and for ARBs 887 (s.d. 277), 693 (s.d. 227) by DDD and reimbursements, respectively. The market share of ACE inhibitors was negatively associated with the concentration in ARBs (p<0.001, p<0.001), and marginally positively associated with the concentration in ACE inhibitors (p=0.05, p=0.09), by DDD and reimbursements, respectively.   CONCLUSIONS: The relative market share of ACE inhibitors and ARBs is cross-linked with concentration of drugs within each of these two classes.