OBJECTIVE: To evaluate the acceptability and effectiveness of a “guided prescription” form implementing locally developed guidelines for preventive anticoagulation. METHODS: Thirty internal medicine wards were randomly assigned to either intervention (I) or control (C) group. I-group physicians were asked to use the guided prescription form whenever they considered prescribing preventive anticoagulation over a 2-month period. One-day surveys of all patients hospitalized in I and C wards were conducted before and after this intervention period. Physicians were also asked to rate independently whether preventive anticoagulation was required for a series of isolated or combined risk factors (1-9 scale), and to assess several attributes of the form (4-degree Likert scale). RESULTS: Of 313 completed prescription forms collected from I wards during the intervention period, 262 (84%) followed recommendations; an explicit “other” option was used for 36 additional patients (12%). Before and after surveys showed an overall decrease from 41% (146/358) to 33% (121/365) (95% CI: -11% to –4%) in the prevalence of preventive anticoagulation in I wards, while prescriptions remained stationary at 38% (154/402 and 134/352) in C wards. While scores assigned to risk factors that did not justify preventing anticoagulation according to the guideline tended to decrease in both groups, this was more marked in the I group. Almost all physicians in both groups agreed that using this form would help improving preventive anticoagulant prescriptions. Nevertheless, 8/45 in the C group and 15/46 in the I group thought that it might hinder prescribing treatments that could benefit patients. CONCLUSION: Further analyses are underway to confirm that changes observed in I group prescriptions reflect improved agreement with the guidelines, and to assess variations across wards and/or for different risk factors or combinations, reflecting possible differences in supervision by senior physicians, or demonstrating the need to improve the documentation of specific recommendations.