Medication errors are one of the most common types of preventable adverse events in ambulatory care, but detecting them is challenging because, unlike hospitalized patients, ambulatory patients are not regularly monitored between visits. Telephone interactive voice response systems (IVRS), which place automated phone calls to patients, have been investigated as a means of detecting adverse events after discharge. This study reports on the feasibility of using IVRS to detect adverse drug events after patients received a new prescription from their primary care physician. The authors identified several limitations: the system only reached 70% of patients despite multiple events, and elderly patients in particular had more technical difficulty with the calls and were less likely to complete them successfully. Further refinement is needed before automated mechanisms can be used to reliably detect outpatient safety problems.