Aim of this study is to determine the setting, causes, and the harm of medication errors (ME) which are notified by Primary Health Care. Setting: Primary Care Regional Health Service of Madrid. 2016. Descriptive and cross-sectional study. All ME (1,839) which were notified by Primary Care Centres by notification system of safety incidents between January 1st 2016 and November 17th 2016. Setting, real harm, potential harm, and cause of error. These items were classified by one researcher. Concordance was checked with another researcher. Just under half (47%) (95% CI: 44.8%-49.3%) of ME occurred in Primary Care Centre, 26.5% (95% CI: 24.5%-28.6%) of ME were patient medication errors, and 27.5% (95% CI: 24.1%-30.8%) of ME were potential severe harm errors. Prescribing errors were the cause of most ME in Primary Care Centre 27.4% (95% CI: 24.4%-30.4%). Communication between patients and doctors were the cause of most patient medication errors 66% (95% CI: 61.8%-70.2%). Patient mistakes and forgetfulness were also causes of patient medication errors. Half of all mediation errors hppened at Primary Care Center while one quarter of them were patient medication errors. One quarter of all ME were potential severe harm errors. The main causes were prescribing errors, failure of communication between patients and doctors, and patient mistakes and forgetfulness. Prescribing aid systems, communication improvements and patients aids should be implemented.