To meet growing demand for oral anticoagulation worldwide there has been increased dependence on computer-assistance in dosage although the safety and effectiveness of any of the individual computer-assisted dosage programs has not previously been established. This randomised multicentre clinical end-point study assessed a new version of the PARMA 5 program. It compared PARMA 5 safety and effectiveness with manual dosage by experienced medical staff at 19 centres with a known interest in oral anticoagulation. Target recruitment was 8000 patient-years, randomised to medical staff or PARMA-5 assisted dosage. Safety and effectiveness of the PARMA 5 program was compared with manual dosage. A total of 10,421 patients were recruited (15,369 patient-years) in the 5-year study. International normalised ratio (INR) tests numbered 167,791 with manual and 160,078 with PARMA 5 dosage. With parma 5 there was overall a non-significant reduction in clinical events but in the 2542 patients with deep vein thrombosis/pulmonary embolism, clinical events were significantly reduced (P = 0.005). Success in achieving 'time in target INR range' was also significantly greater with PARMA 5 compared with the dosage by experienced medical staff. This study demonstrated the safety and effectiveness of PARMA 5-assisted dosage.