Appropriate use of drugs is an essential element in achieving quality of health and medical care for patients and the community as a whole. This study aimed to measure the drug prescribing performance of primary health care centres in Eastern province, Saudi Arabia, using the WHO/International Network of Rational Use of Drugs core drug prescribing indicators. In a retrospective cohort study 10 health centres were selected using systematic random sampling. A total of 1000 prescribing encounters were investigated from January to December 2010. Mean values were number of drugs per encounter 2.4 (optimal ≤ 3), drugs prescribed by generic name 61.2% (optimal 100%), encounters with antibiotic prescribed 32.2% (optimal ≤ 30%), encounters with injection prescribed 2% (optimal ≤ 10%) and drugs prescribed from the national essential drugs list or facility formulary 99.2% (optimal 100%). An overall index of rational drug prescribing was calculated and applied to rank the health centres for benchmarking.