Data on 224 diabetic patients were collected to determine the effect of tolbutamide (Orinase) on coumarin anticoagulant therapy and prothrombin times, using insulin injection-treated diabetic patients as controls. This retrospective survey indicates that there is no potentiating or inhibitory effect of tolbutamide, as compared to insulin, on anticoagulant therapy. When tolbutamidetreated patients were compared, there was no statistically (or clinically) significant difference at the 0.05 level or beyond, with respect to average initial dosage or maintenance dosage of anticoagulant, and day 4 and average prothrombin time. Likewise, there was no significant difference between treatment groups in the number of patients who were "adequately" maintained on anticoagulants. In conclusion, this study fails to demonstrate a difference in effect between tolbutamide and insulin on anticoagulant therapy, either potentiating or inhibitory.