Objective. To determine the effect of pre-emptive ambulance distribution based on the implementation of a real-time, Internet-accessible emergency department (ED) workload schematic andprehospital Australasian Triage Scale (ATS) allocations on ambulance diversion in Western Australia. Methods. Comparison of July–December 2002 andJuly–December 2003 metropolitan Perth ED cubicle occupancy, ambulance diversion, ambulance distribution, andambulance unloading delays at four inner andfour outer metropolitan EDs. Results. Ambulance diversion fell from 1,788 hours in 2002 to 1,138 hours in 2003 (p <0.001) despite an increase in mean weekly ED cubicle occupancy from 31 patients (95% confidence internal [CI] 29–33) in 2002 to 39 patients in 2003 (95% CI 36–43, p <0.001). Inner metropolitan ED ambulance attendances fell 2.7% from 27,475 in 2002 to 26,743 in 2003, andouter metropolitan correspondingly rose from 5,877 to 6,628 ambulance attendances (p <0.001). Unloading delays were similar in 2002 (219, 0.66%) and2003 (223, 0.67%, p = 0.84); however, median duration of unloading delays increased from 38 minutes (interquartile range [IQR] 18–68) in 2002 to 50 minutes (IQR 25–108) in 2003 (p <0.001). Conclusions. The implementation of pre-emptive ambulance distribution using Internet-accessible ED information andprehospital ATS allocations was associated with reduced ambulance diversion, probably consequent upon the redistribution of ambulances from inner to outer metropolitan EDs. The rise in ED cubicle occupancy between the study periods suggests that this approach to reducing ambulance diversion should be viewed only as complementary to direct efforts to reduce ambulance diversion by improving hospital inpatient flow andthe balance between acute andelective hospital inpatient accommodation.