Several studies have found reduced use of recommended medications after myocardial infarction (MI) in patients with impaired kidney function. However, the reasons for such undertreatment are not well understood. A total of 1380 Medicare patients who survived at least 90 d after MI and had prescription drug coverage through Pennsylvania's medication assistance program for the elderly were studied. Filled prescriptions were used to assess use of angiotensin-converting enzyme inhibitors (ACEI), angiotensin receptor blockers (ARB), beta blockers, and statins within 90 d of MI. Patients' demographics, comorbidities, and health care utilization before MI also were ascertained. We used logistic regression to test the association between kidney function and postdischarge use of each medication. Overall, 619 (45%) patients filled a prescription for a beta blocker, 675 (49%) received an ACEI or ARB, and 406 (29%) received a statin after discharge but within 90 d after their admission for MI. Reduced kidney function was associated with both lower beta blocker and statin use (P = 0.01 and P = 0.002, respectively), but after multivariate adjustment, these associations disappeared (P = 0.23 and P = 0.62, respectively). Use of ACEI or ARB was nearly half in patients with estimated GFR <30 ml/min compared with patients with better kidney function in univariate and multivariate analyses (P < 0.001). Analyses using serum creatinine measurements rather than estimations of GFR yielded similar results. Differences in other characteristics such as age, rather than kidney function, may be responsible for much or all the reported reduction in use of preventive medications after MI seen in patients with chronic kidney disease.