The Palestinian Ministry of Health introduced an essential medicines list (EML) in 2000 to improve rational use of medicines and contain costs. We have examined the effects of the EML in the Palestinian healthcare public sector. We obtained data on prescription patterns for medicines from 3570 prescriptions given during outpatient visits in 17 healthcare facilities in the West Bank from 1997 to 2003. We analyzed the prescriptions to measure rational use. We modeled indicators of rational use as a function of the EML and 16 health center indicator variables. The EML was effective in shifting all prescribing indicators toward standard values. To improve rational medicine use, treatment protocols for the most common diseases and continuous education for medical staff is required.