This prospective cohort study looked at prescribing errors in four adult primary care practices where both handwritten and basic electronic prescription support were used. Eight percent of prescriptions had an error. Practices with only "basic" computer support (such as no or optional allergy and drug interaction checks) were no safer than those with handwritten prescriptions. More advanced decision support (i.e., requiring complete prescriptions and providing default dosing and frequencies) could have prevented most errors.