Background: The ability to use printed material to function in society (literacy) and to handle basic numerical concepts (numeracy) may have implications in patients' ability to follow dosing schedules. We examined literacy and numeracy skills among patients on warfarin and explored their association with anticoagulation control. Methods and Results: Patients older than 50 years attending two anticoagulation management units were prospectively enrolled. We measured literacy, numeracy, and international normalized ratio (INR). During a 3-month follow-up period, we calculated the variability of the INR and the amount of time a patient's INR was within his or her therapeutic range, variables associated with bleeding and effectiveness. Among 143 patients, only 75 (52.4%) were able to read health-related words at the eighth grade level or less. Patients' self-reported grade completed was higher than the measured literacy grade level (κ = 0.21). While 79.0% had completed at least eight grades, only 47.6% had a score at that grade level. Sixty-nine patients answered none or correctly answered fewer than two of the six numeracy questions (48.3%). The INR variability was higher among patients with lower literacy (P = 0.009) and lower numeracy skills (P = 0.004). The time in range was similar among patients at different literacy levels (P = 0.9). Patients with lower numeracy level spent more time above their therapeutic range (P = 0.04) and had a trend of less time spent in range (P = 0.10). Conclusions: Low literacy was prevalent among study patients taking warfarin. Low literacy and numeracy were associated with measures of poor anticoagulation control.