The safety of the more than 45% of the United States population that ingests at least 1 prescription medication daily is compromised by more than 50 million medication errors occurring annually in pharmacies nationwide. This presents a dangerous problem because medication errors lead to adverse drug events and death if they are not discovered before patients ingest the medications. The goal of this single site prospective observational study conducted at a retail pharmacy in Carroll County, Maryland, was to measure the frequency and sources of medication errors in a retail setting and to determine the proportion of medication errors that reach the patient before being discovered. The frequency of dispensing errors in the pharmacy studied was 0.26%. Of the total 33 errors noted, 12 were discovered by patients. Attention to sources of error such as distractions to pharmacy employees, technicians' lack of knowledge concerning pharmaceutical products, and pharmacists' failure to view hard copy prescriptions during verification are needed to increase patient safety. These findings suggest the need for increased monitoring of dispensing errors in community pharmacies.