Copies of 1814 prescriptions written by 20 residents were reviewed to determine prescribing patterns and the incidence of prescription-writing errors. An average of 0.69 prescriptions were written per patient visit at an average cost (to the pharmacist) of $13.35. Over one third of all prescriptions were written using the generic name, and "dispense as written" was specified on only 3.25%. On average, 21% (n = 373) of all prescriptions collected contained at least one prescription-writing error. Errors were characterized as omissions (6%), unfulfilled legal requirements (1%), incomplete directions (1%), dose or direction errors (3%), unclear quantity to be dispensed (3%), or prescriptions written for nonprescription products (5%). A correct diagnosis and treatment plan can be undermined by a written prescription that is incorrect or miscommunicates the intention of the prescriber.