To determine the frequency of avoidable adverse events (AAEs) in Primary Care (PC). Retrospective cohort study. Family medicine and paediatric clinics in Andalusia, Aragon, Castilla-La Mancha, Catalonia, Madrid, Navarre, and Valencia. A review was performed on a designated sample of 2,397 medical records (95% confidence level and 2% accuracy). The sample was stratified by age group as regards the frequency of physician consultations and considering equal distribution of male and female patients. Number and severity of identified AAEs from February 2018 to September 2019. A total of 2,557 medical records were reviewed (1,928, 75.4% of adult patients, and 629, 24.6% paediatrics). A total of 182 (7.1%, 95% CI 6.1-8.1%) AAEs that affected 168 patients were identified, which included 7.6% (95% CI 6.4-8.8%) in adults and 5.7% (95% CI 3.9-7.5%) in paediatric patients. The number of AAEs in women was higher than in men (P = 0.006). The incidence of AAEs in boys and girls was similar (P = 0.3). Permanent damage was caused by AAEs in 6 (4.1%) adult patients. Seeking formulas to increase patient safety in PC should remain a priority objective, particularly in female patients and in paediatrics. One in 24 AAEs causes serious and permanent damage in adults.