Few data are available on factors associated with low adherence or early clopidogrel discontinuation after percutaneous coronary intervention (PCI). Patients (n = 284) were evaluated before hospital discharge after PCI to identify factors associated with low adherence to clopidogrel 30 days later. Adherence to daily medications before PCI was assessed using the 8-item Morisky Medication Adherence Scale (MMAS-8) and categorized as low (score <6), medium (score 6 to <8), or high (score 8). Low adherence to clopidogrel was defined as MMAS-8 score <6 (n = 21) or having discontinued clopidogrel (n = 11), which was ascertained during a 30-day interview after PCI. At 30 days after PCI, 11% of patients had low adherence to clopidogrel. Odds ratios (95% confidence intervals [CIs]) for low adherence to clopidogrel were 3.78 (1.09 to 13.1), 3.06 (1.36 to 6.87), 2.46 (0.97 to 6.27), and 3.36 (0.99 to 11.4) for patients who before PCI reported taking smaller doses of medication because of cost, had difficulty filling prescriptions, had difficulty reaching their primary physician, and were not comfortable asking their doctor for instructions, respectively. Odds ratios (95% CIs) for low clopidogrel adherence after PCI in patients with medium and low versus high adherence to daily medications before PCI were 6.13 (1.34 to 28.2) and 10.9 (2.46 to 48.7), respectively. The c-statistic associated with MMAS-8 scores before PCI for discriminating low clopidogrel adherence at 30 days after PCI was 0.733 (95% CI 0.650 to 0.852). In conclusion, adherence to daily medications before PCI may be a useful indicator for identifying patients who will have low clopidogrel adherence after PCI. © 2011 Elsevier Inc. All Rights Reserved.