Medication errors can cause a variety of adverse drug events but are potentially preventable. This cross-sectional study analysed all medication prescriptions from 5 public and 5 private primary health care clinics in Riyadh city, collected by simple random sampling during 1 working day. Prescriptions for 2463 and 2836 drugs from public and private clinics respectively were examined for errors, which were analysed using Neville et al.'s classification of prescription errors. Prescribing errors were found on 990/5299 (18.7%) prescriptions. Both type B and type C errors (major and minor nuisance) were more often associated with prescriptions from public than private clinics. Type D errors (trivial) were significantly more likely to occur with private health sector prescriptions. Type A errors (potentially serious) were rare (8/5299 drugs; 0.15%) and the rate did not differ significantly between the 2 health sectors. The development of preventive strategies for avoiding prescription errors is crucial.