Computer-generated prescription drug purchase records for ambulatory patients receiving oral anticoagulants (OAC) were studied for concomitant use of other drugs which have been reported to induce clinically significant interactions. One third of 479 patients taking OAC were exposed to a potentially interacting drug at some time during this 6-month period. The percentage of patients with drug interaction exposure correlated directly with total drug use (p < 0.0005). There were no significant differences when interaction exposure rates were compared in the cases of single: multiple pharmacy and single: multiple physician-patient groups. Warfarin was the most common anticoagulant (>95%) and barbiturates the most common interacting drug.