Objective: To investigate whether treatment initiated with an angiotensin-converting enzyme inhibitor (ACE-I) or an angiotensin II receptor blocker (ARB) for patients with ischemic heart disease, hypertension, or diabetes causes a reduction in hemoglobin (Hb) levels. Patients and Methods: This was a retrospective cohort analysis using the computerized database of a large health maintenance organization. Included were all adults with a first purchase of an ACE-I, an ARB, or a calcium channel blocker (CCB) between January 1, 2004, and December 31, 2009, defined as the index date. Measures of Hb levels before and 1 year after the index date were reviewed, and the change was calculated. All the analyses were stratified by pharmaceutical class. The main exposure variables were the proportion of days covered (PDC) by these drugs and the mean enalapril dosage (for enalapril users only). Results: Levels of Hb before and after treatment were available for 14,754 patients taking ACE-Is, 751 taking ARBs, and 3087 taking CCBs. A high PDC was significantly associated with greater yearly reductions in Hb levels compared with a low PDC for CCB use, but was more pronounced for ACE-I and ARB use. A high PDC was also associated with a higher odds of developing anemia in ACE-I users (odds ratio [OR], 1.59; P<.001) and ARB users (OR, 2.21; P=.05). In nonanemic enalapril users, every 10-mg increment in daily dose was associated with an OR of 1.45 for the development of anemia (P<.001). The association remained after excluding nonadherent patients. Conclusion: Levels of Hb are reduced during the first year of use of ACE-Is and to a lesser extent with use of ARBs. This association is dose dependent and is not explained by patient adherence.